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Full text of "Contributions to solar physics. I. A popular account of inquiries into the physical constitution of the sun, with special reference to recent spectroscopic researches; II. Communications to the Royal society of London, and the French Academy of sciences, with notes"

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CONTRIBUTIONS 



TO 



SOLAR PHYSICS. 



CONTRIBUTIONS 



TO 



SOLAR PHYSICS. 



I. 

A POPULAR ACCOUNT OF INQUIRIES INTO THE 
PHYSICAL CONSTITUTION OF THE SUN, 
WITH SPECIAL REFERENCE TO RECENT SPECTROSCOPIC 

RESEARCHES ; 

II. 

COMMUNICATIONS TO THE ROYAL SOCIETY OF LONDON, 
AND THE FRENCH ACADEMY OF SCIENCES, 

WITH NOTES. 



BY 



J. NORMAN LOCKYER, F.R.S. 



MACMILLAN AND CO. 

1874. 

» ^ ■ 

[ The Right of Translation and Reproduction is reserved, ] 



LONDON : 
K. CI.AV, SONS, AND TAVLOI, FRINTRKS 
BRRAD STREET HILL. 



; t 
1 



1 t 



V- 




This Book is Dedicated to Balfour Stewart and 
P. C. Jules Janssen, Encouraged by One 
Friend I undertook the Work which has brought 
nie The Other. 



PREFACE. 

The present volume has grown out of an intention 
I formed about a year ago, to publish the Papers I 
had communicated to the Royal Society, dealing with 
the new facts which a new method of inquiry had 
revealed to us. I formed this resolution because, 
doubtless owing to their being scattered among the 
publications not easily procurable of a learned body, 
these Papers were evidently unread by some who were 
actually engaged, as well as by many who were 
merely interested, in the inquiry. 

It next struck me that it would be wise to re- 
lieve the great — I fear too-great — terseness of these 
Papers by introducing into the same volume three 
Lectures I had also published in the Proceedings of the 
Royal Institution, giving an account of the first out- 
come of my inquiries, and of the results of the two 
Eclipse Expeditions which I had accompanied to the 



viii PREFACE. 



Mediterranean and India. After I had determined 
to appeal by the publication of these to a wider public, 
I chanced to fall upon the notes of a course of four 
other Lectures on the Sun which I had also given 
in the Royal Institution, but which remained unpub- 
lished, and were almost forgotten. This decided me 
to attempt to make the story of the work more com- 
plete, and to add to it the information necessary for 
the general reader both as to the telescopic and spec- 
troscopic sides of the inquiry, by means of these notes 
and some of the Essays which I had published at 
different times during the last ten years in The Reader, 
Macmillatis Magazine (these were written in conjunc- 
tion with my friend Dr. Balfour Stewart), Nature^ the 
Times^ Daily News^ and other periodicals. 

This, then, is the origin of the work in its present 
shape. 

In the First Part I have attempted to give a 
sketch of the various inquiries into the Physical 
Constitution of the Sun, and I have not hesitated 
to alter the arrangement of the four Lectures, and, 
in some cases, the Essays to which I have re- 
ferred, so as to make the story a continuous one. 
I have not only largely added to the parts dealing 
with Spectrum Analysis, but I have given a body of 



PREFACE. ix 



information on this new science in its special relation 
to its solar applications which I hope may be found 
of value. 

The Second Part, which consists of my Papers 
communicated to the Royal Society and to the French 
Academy of Sciences, of course is given verbatim^ 
with the exception of the references to the illustrations, 
many of which have been transferred to the First Part 
of the book. To these Papers I have added Notes, 
on some of the more fundamental outcomes of the 
research, in which I have attempted to show the 
relations of my observations to those made by others ; 
on the objections urged against some of the conclu- 
sions to which I had been led ; and on the new facts 
acquired to Science by the labours of my fellow- 
workers. On this latter point, however, I have been 
compelled not only to give slight references where I 
could have wished to have given full details, but to 
cancel much matter actually written in order to keep 
the volume within convenient limits. 

I must, however, thank Professors Young and 
Respighi for permitting me to largely increase the 
value of my book by referring at the length I have 
done to their observations. The faithful translation 
of the memoirs of the illustrious Italian observer here 



PREFACE. 



given, has been obligingly placed at my disposal by 
Mrs. G. M. Patmore. 

Whilst the latter portions of this work have been 
passing through the press, a very important discussion 
has been carried on in the French Academy of 
Sciences, in which nearly every question raised by 
the new method of research in solar physics has been 
debated. I much regret that it has been impossible 
to include a notice of it in the present volume. It 
may, however, be remarked as satisfactory to English 
Science, that M. Faye, abandoning the theory of spots 
of which an account will be found in Chapter IV., 
has virtually adopted, in the essential point, that pro- 
posed by the English Observers. 

My best thanks are due to the Proprietors of the 
Journals I have named for the readiness with which 
permission has been granted me to reprint ; while in 
the matter of illustrations I have to express my 
obligations to the Council of the Royal Society ; to 
Ur. Schellcn, of Cologne, for the use of several wood- 
cuts illustrating my own observations, the original 
drawings of which I sent over to him in 1869 ; to Mr. 
Westermann, of Leipsig, who added others to them ; 
and to Professor Roscoe, who has allowed me the 
use of several which appear in his ** Spectrum Analysis." 



PREFACE, xi 



I am much indebted to Mr. Cooper for the care he 
has taken in the preparation of the plates and new 
woodcuts . 

In the compilation of the Notes, and in revision 
of proof-sheets, my assistant Mr. R. J. Friswell has 
given me most valuable aid. 

Up to the present time the spectroscopic exami- 
nation of the sun has been regarded as the work 
of the astronomer and physicist, rather than of the 
chemist; and in England, though happily not abroad, 
many professional astronomers and physicists regard 
it, as a rule, as a matter of tenth-rate importance. I 
am sanguine enough to hope that, as time goes on, 
breadth of mind will take the place of the present 
more than apathy, and that chemists also will more 
generally interest themselves in, and aid, an inquiry 
from which, if I am not mistaken, they will learn much. 

I cannot conclude this Preface without stating that 
had it not been for the aid afforded me by that admir- 
able, but too little known, institution, the Government 
Grant Fund, and by my friend Dr. Frankland, who 
joined me in a branch of the research and generously 
placed his laboratory at my disposal, my observations 
would probably never have been made. Further, I 



Aii PREFACE, 



admit and lament the incompleteness of them and of 
the book to which I have now consigned them. I know 
that the work I have attempted to forward requires 
a man who can give himself entirely up to it, while, 
less fortunate than many lovers of Science, the only 
time I have had to devote to these inquiries has 
consisted of fragments snatched from the leisure left 
me by my official duties. I have, however, the satis- 
faction of knowing that the method of observation 
which I have had a share in originating is rapidly 
taking root in other lands, and that it is being recog- 
nized as national work which the Janssens, Youngs, 
Respighis, Zollners and Secchis of the future will carry 
on without break, for the instruction and benefit of 
mankind. 

J. Norman Lockyer. 
Sept. tth^ 1873. 



CONTENTS. 



PART I. 
A POPULAR ACCOUNT OF ANCIENT AND MODERN SUN-WORK. 

CHAf. FAOE 

1.— What is a Sun? i 

Lecture delivered at the Royal Institution in March zSja 

II.— On the Telescopic Appearance of the Sun .... 13 

In part reprinted (with alterations) from the Reader^ vol. iii. p. 79, 
1864. The remainder consists of a paper communicated to the Royal 
Astronomical Society in 1865, and printed in Monthly Notices^ vol. 
XXV. p. 236. 

III.— Mr. Carrington's Researches on Solar Spots ... 32 

Reprinted (with additions) from the Retuitr, vol. iii. p. 581 1864. 

IV.— M. Faye's First Theory of the Physical Constitution 

OF THE Sun 44 

Reprinted from the Reader^ vol. v. p. 107, 1865. 

v.— M. Faye's First Theory of the Physical Constitution 

OF THE Sun (continued) 51 

Reprinted from the Reader , vol. v, p. 140, 1865. 

VI.— The Sun as a Type of the Material Universe ... 63 

An article written in conjunction with Dr. Balfour Stewart, F-R.S. 
Reprinted from Macmillan's Magazitie^ vol. xviii. p 319, 1&68. 

VII.— The Place of Life in a Universe of Energy .... 85 

An article written in conjunction with Dr. Balfour Stewart, F.R.S. 
Reprinted from Macmillan*s Magazine, vol. xviii. p. 3x9, 1868. 



xiv CONTENTS, 



CHAI'. PAGK 

VIII.— The Place in Science of the New Method. ... 104 

From the Times, Jan. 9th, 1869, with additions from an Article in 
Mactnillan^i Ma^azine^ vol. xix. p 330, Jan. 1869. 

IX— The Birth of Spectrum Analysis 132 

Lecture delivered at the Royal Institution in April 1870, revised in the 
light of recent work. 

X.— The Modern Spectroscope 154 

XI.— Radiation and Absorption 169 

XII.— History of the Application of the Principles of 

Spectrum Analysis to the Solar Spectrum ... 185 

XIII.— Results obtained by the Old Method 196 

XIV.— The First Results of the New Method 209 

A Lecture delivered at the Royal Institution of Great Britain, Friday 
evening, May aSth, 1869. 

XV.— The American Eclipse, 1869 240 

From Nature^ vol. i. p. 14, 1869, and Proc. R. S , vol. xviii. p. 179. 

XVI.— The Mediterranean Eclipse, 1870. A Letter from 

Venice , 258 

From Satarr^ vol iii. p. 321, 1871. 

XVII.— The Mediterranean Eclipse, 1870 {continued). More 

Light 270 

From XiitMrr, v«)l. iii. p. 321, 1871. 

XVIII.— The Story of the Corona in connection with the 

Mediterranean Eclipse 278 

A Lecture delivered at the Royal Institution of Great Britain, Friday 
evening, Marrh 17, 1871. 

XIX.— The Atmosphere of the Sun 311 

The Rede I.*cturc. 1871, delivered in the Senate Hou^e, Cambridge, 
on May 34, 187T. 

XX.— The English Eclip.se Expedition, 187 i - 

I. Preliminary 332 

Frum Xa/urr, vol. iv. p. 197, 1873. 



CONTENTS, XV 

CHAP. FAGK 

XXI.— The English Eclipse Expedition, 1871 {continued)-- 

II. The Berul Party 339 

Extracted from a letter in the Daily News, January 15th, 1872. 

XXIL— The English Eclipse Expedition, 1871 {continued)— 

III. A Letter from Ootacamund 347 

From Nature, vol v. p. 2x7, 1872. 

XXIII.— The English Eclipse Expedition, 1871 {continued)— 

IV. Intelligence from the Other Parties .... 356 

From Nature, vol. v. p. 259. 

XXIV. — The English Eclipse Expedition, 1871 {continued) — 

V. General Statement of the Methods used, and 

Results obtained. By the Expedition .... 362 

A Lecture delivered at the Royal Institution of Great Britain, on 
Monday, March 22, 1872. The chief results obtained by the Expedi- 
tion have been taken from the ad interim Report presented to the 
British Association Meeting in Brighton. 

XXV.— Three Years' Work with the New Method ... 381 

Re%'lsed from the short-hand notes of two Lectures delivered before the 
Literary and Philosophical Society of Newcastle-upon-Tyne, in 
October 1872. 

XXVI.— The Meteorology of the Future ....... 424 

From Nature^ vol vii. p. 98. 



PART IT. 

COMMUNICATIONS TO TKK ROYAL SOCIETY OF LONDON AND 
TO TH?: FRENCH ACADEMY OF SCIENCES. 

PAGE 

1. — Observatory Work :— 

Spectroscopic Observations of the Sun. No. I. . . 435 
Spectroscopic Observations of the Sun. No. II. 

Brie/ Announcement of the Success of the New Method. . 439 

Complete Account 445 

Spectroscopic Observations of the Sun. No. III. . . 477 

Spectroscopic Observations of the Sun. No. IV. . . 488 



xvi CONTENTS, 



PACK 

Spectroscopic Observations of the Sun. No. V. . . 493 

Discussion with Father Secchi in the " Comptes Rendus " 0/ 
the French Academy of Sciences 500 

Spectroscopic Observations of the Sun. No. VI. . . 516 

II.— Laboratory Work. 

Preliminary Note of Researches on Gaseous Spectra 
IN Relation to the Physical Constitution of the 
Sun. (In conjunction with Dr. Frankland) ... 525 

Researches on Gaseous Spectra in Relation to the 
Physical Constitution of the Sun, Stars, and 
NEBULiE. No. II. (In conjunction with Dr. Frank- 
land) 530 

Researches on Gaseous Spectra in Relation to the 
Physical Constitution of the Sun, Stars, and 
Nebulae. No. III. (In conjunction with Dr. Frank- 
• land) 5^2 

Researches in Spectrum Analysis in connection with 
THE Spectrum of the Sun. No. 1 534 

Researches in Spectrum Analysis in connection with 
THE Spectrum of the Sun. No. II 555 

III.— Notes. 

A. — Spot Phenomena and Theories 561 

B.— Mr. Huggins and the New Method 570 

C— Absorption at the Limb 574 

D.— Methods of Viewing the Forms of the Promi- 
nences 578 

E.— Structure, Height, and Composition of the 

Chromosphere 585 

P.— Prominences on the Sun 587 

G. — Changes of Wave-length 589 



> J 



CONTENTS. xvii 

PAGE 

H.— List of Chromospheric Lines 602 

L— The Line 1474 (Kirchhoff) . 621 

K.— The Continuous Spectrum at the Base of the 

Chromosphere 622 

L.— The Classification of Prominences 624 

M.— Connection between Prominences, Spots, and 

FACULiB 628 

N.— The Expansion of Linf.s at the Base of the 

Chromosphere 635 

O. — Welling up of Magnesium and Iron 637 

P.— Solar Outbursts and Magnetic Storms .... 638 

Q. —Simplification of Spectra by Reduction of 

Pressure 640 



APPENDICES.* 

L— Instructions to Observers (Eclipse of 187 i) 649 

II. — Professor Respighi's Memoir on the frequency and 
distribution of the Prominences and their Periodical 
Vi^iATiONS 654 

i The Appendices referred to on pages 142, 252, and 262 have been omitted for 
want of space. 



LIST OF ILLUSTRATIONS. 



COLOURED AND LITHOGRAPHIC PLATES. 



Frontispiece.~The Spectroscopic Corona, 1871. 

Plate L— The various kinds of Prismatic Spectra . 

Plate IL— Solar Prominences 

Plate IH. — The Photographic Corona, 1869 .... 

Plate IV.— The Observatory at Bekul 

Plate V.— The Photographic Corona, 187 i .... 

Plate VI.— Map of the Long and Short Lines of some 

Metallic Elements to face 532 



to face 


fACF. 
172 


>» 


209 


>» 


240 


»> 


343 


>» 


375 



WOODCUTS. 



PIG. 

1. Portrait of Galileo 

2. Explanation of sun-spots on 

Wilson's hypothesis . . . 

3. HerschePs theory of sun-spots . 

4. Herschel's theory of sun-spots . 

5. Earlv sun-work. Schciner's 

Iieliotropium Teleoscopicum . 

6. Cvclonic sun-spot (Secchi.) 

7. Changes in sun-spots. The great 

sun-spot of 1865. (Howlett. ) 

8. Facuhe surrounding a spot, as 

seen near the sun's edge. 
(Noble.) 

9. Sketch of sun-spot, completed 

about iih. 40m. a.m., April 

2, 1865 

la The same at I2h. 30m. . . . 

11. The same at 1 2h. 55m. . . . 

12. The same at I2h. 30m. . . . 



PAOK FIG. PAGE 

7 ; 13. Sun spot. (Secchi.) .... 29 

14. Sun spot. (Secchi.) .... 30 

9 ] 15. The earth, seen from the sun 

1 1 (Vernal equinox) • • • • 35 

12 16. The earth, seen from the sun 

(Summer solstice) .... 35 

13 17. The earth, seen from the sun 

16 (Autumnal equinox) ... 36 

18. The earth, seen from the sun 

17 (Winter solstice) .... 36 

19. The sun, seen from the earth. . 37 

20. Explanation of sun-spots on 

18 KirchhoflTs hypothesis . . . 48 

21. The eclipsed .sun 75 

22. Path of moon's shadow during 

25 Eclipse of i860 no 

27 23. Total Eclipse, Aug. 18, 1868. 

27 (Observations at Aden.) Com- 

28 mencement of totality . . .112 



LIST OF ILLUSTRA TIONS, 



XIX 



riG. PAGB FIG. 

24. Total Eclipse (Indian), Aug. 18, 54. 

1868, near end of totality . . 1 13 

25. Total Eclipse (Indian), Aug. 18, 55. 

1868 : end of totality . . .114 

26. Eclipse of 1868. (Corona) . . 120 56. 

27. Copy of Tennant's photograph : 57. 

iDeginning of totality . . .121 58. 

28. Copy of Tennant's photograph : 

end of totality 122 59. 

29. Synoptic view of Tennant's photo- 60. 

graphs 123 61. 

30. Tennant's reflector used for 

photo^phy 124 62. 

31. Geometrical form of the prism . 132 

32. Prism mounted on a stand . .133 

33. Light passing through a plate of 63. 

glass 134 64. 

34. Images of objects seen through 

prisms »35 65. 

35. Copy of Kepler's diagram . .137 

36. Path of the refracted ray . . . 138 66. 

37. Refraction of lighL Apparent ele- 

vation of the bottoms of vessels 1 38 

38. Decomposition of light by the 67. 

prism 140 

39. Unequal refrangibility of two 68. 

diflerently coloured lights . . 141 69. 

40. Recomposition of white light by 

means of a second prism . .142 70. 

41. Recomposition of white light by 71. 

means of a lens 143 

42. Recomposition of white light by 72. 

means of a rapidly revolving 
disc, coloured in sectors . .144 

43. The first observation of Fraun- 73. 

hofer's lines 147 

44. Reduced copy of Fraunhofer's 74. 

map of the lines in the solar 
spectrum 150 

45. Chemical or student's spectro- ■ 75. 

scope 157 

46. Spectroscope with two prisms . 157 , 76. 

47. Steinheil's form of four prism ! 

spectroscope 158 77. 

48. Bunsen's first form of spectro- 

scope 158 78. 

49. Spectroscope with reflected scale. 159 

53. Steinheil's slit, showing reflect- 79. 

ing prism 161 J 

51. Path of light through reflecting 

prism and into the slit . . . 161 80. 

52. Spectroscope showing the use of 

comparison spectra and a re- 
flected scale 161 81. 

5j. Direct-vision prism with three 

prisms 162 



pAoe 

Direct-vision prism with five 
prisms 162 

Telespectroscope of small dis- 
persion. (Huggins.) . . . 163 

Side view 164 

Plan 164 

Direct vision star spectroscope 
(Secchi.) 165 

Sun spectroscope (Lockyer) . .166 

Sun spectroscope (Young) . .166 

Young's sun spectroscope, ar- 
ranged for photography . .167 

Automatic arrangement for secur- 
ing the minimum deviation of 
the observed ray 168 

Electric lamp 170 

Electric lamp arranged for throw- 
ing a spectrum on a screen . 171 

Coloured flame of salts in the 
flame of a Bunsen's burner . 173 

Base of ordinary form of Bunsen's 
burner, showing holes to admit 
air . 173 

Mitscherlich's arrangement for 
flame spectra 174 

Enlarged view of wick . . .174 

Herapath's blow-pipe. View and 
section 175 

Induced current spark . . . .175 

Figure showing how the jar is 
connected with the spark stand. 1 76 

Becquerel's arrangement for study- 
ing the spark spectra of va- 
pours 177 

Geissler's tube, showing electric 
discharge 178 

Wheatstone's map of the spectra 
of the vapours of some of the 
elements (1835) 179 

Method of observing the absorp- 
tion of a vapour 180 

Vessel with glass sides for study- 
ing the absorption of liquids . 181 

Absorption spectra of iodine and 
nitrous fumes 182 

Absorption of magenta and 
blood 183 

Absorption of various thick- 
nesses of a solution of the salts 
of chromium. (Gladstone.) . 183 

Absorption of various thick- 
nesses of a solution of potassic 
permanganate. (Gladstone.) . 183 

Coincidence between the bright 
line given out by sodium vapour 
and the 4ark line produced by 



XX 



UST OF ILLUSTRATIONS, 



FIG. PAGE 

the absorption of the light of 
a continuous spectrum by its 
passage through sodium vapour 192 

82. Coincidence of some of the bright 

lines of iron with some of the 
Fraunhofer lines 197 

83. The telluric lines 203 

84. The spectroscope attached to the 

telescope for solar work . .214 

85. Enlarged view of spectroscope . 215 

86. Line C (red), with radial slit . 216 

87. Line D3 (yellow), with radial 

slit 2x6 

88. Line F (blue-green), with radial 

slit 217 

89. Line C, with tangential slit . .217 

90. Spectrum of chromosphere, show- 

mg the different lengths of the 
lines of hydrogen, sodium, and 
magnesium 221 

91. Contortions of F line in a pro- 

minence 222 

92. Prominence observed 14th March, 

1869, iih. 5m 224 

93. The same prominence, Iih.i5m. 225 

94. Spectrum of a sun-spot, show- 

ing the increased absorption of 
the sodium vapour .... 226 

95. Sun-spot. {Secchi.) .... 230 

96. Contortions of F line on disc . ;»33 

97. Contortions of F line on disc, in 

connection with spots and up- 
rushes of bright hydrogen . . 234 

98. Alterations of wave-length in 

prominences 235 

99. Solar cyclone 236 

100. Non-coincidence of bright and 

dark F line 237 

loi. Comparison of b and adjacent 

lines 238 

102. The eclipsed sun, August 1869 . 242 

103. Copy of a photograph of the 

American eclipse of August 7, 
1869 244 

104. General view of the protuber- 

ances, American eclipse, Aug. 

7. 1869 245 

105. Gould's drawing of the corona, 

near the beginning of totality, 

at Burlington 248 

106. Gould's drawing of the corona, 

near the end of totality, at 
Burlington 249 

107. Why the corona was obser%ed 

square 276 

108. Total eclipse, 1851. (Dawes) . 279 



FIG. 

109 

r 
I 10. 

III. 

112. 
"3. 

114. 

115. 

116. 
117. 

118. 
119. 
120. 

121. 

122. 

123. 
124. 



125. 
126. 

127. 



128. 
129. 

130- 

132. 

>33- 

'34- 

135- 
136. 

137. 
138. 
139. 



FAGB 

Total eclipse, September 1858. 

(Lias) 280 

Total eclipse, i860. (Rays ob- 
served by Feilezsch) . . . 282 
Rays observed in the total 

eclipse, 1840 283 

Why the corona was observed 

square 285 

Eclipse of 1868. The corona as 
observed at Mantawalek Ke- 

kee 189 

Gould's drawing of the corona, 
near the beginning of totality, 

at Burlington 290 

Gould's drawing of the corona, 
near the end of totality, at 

Burlington 291 

Eclipse of 187a Photograph of 
the corona taken at S3rracuse . 292 
Explanation of the feeble or 
strong polarization of the 

corona 294 

Copy of Rayet's diagram . . 299 
Copy of Pogson's diagram . . 301 
Track of the moon's shadow, 

eclipse of 1871 333 

The spectrum of hydrogen as 
seen with a straight, a crooked, 

and a ring slit 367 

Young's observation of the rever- 
sal of the sodium lines over a 

spot 389 

Spectroscope, with camera at- 
tached 392 

Cirrus mass of prominences 
observed by Young, Sept 4, 

1869 393 

Small prominence 393 

A small forked prominence . . 393 
Cumulus prominences like the 
fish-mouth in the nebula of 
Orion. (Young) .... 393 
Forms of prominences (Zollner) 393 
Forms of prominences (Zollner) 394 
Forms of prominences (Young) 394 
Forms of prominences (Young) 394 
Rising of a detached fragment . 397 
Filamentous prominence . . . 397 
Streamer prominence .... 398 
Tree-like prominences . . . 399 
The prominence after the ex 
plosion 



The " thunderhead " at 1.40 
The " thunder-head " at 1.55 
Changes in prominences, Oct 7, 
1869. (Young) 401 



399 
400 

400 



UST OF ILLUSTRA TIONS. 



XXI 



FIG. PAOB 

14a Diagram showine how the pro- 
minences are daily recorded. 
(Respighi) 420 

141. My first prominence .... 441 

142. Secchi's first obiervation of the 

chromosphere 461 

143. Arrangement for comparison of 

spectra of different parts of the 
sun 475 

144. Motion forms 490 

145. Motion forms 491 

146. Upnish of bright hydrogen . . 492 

147. Secchi's observations of the C 

line 503 

148. Secchi's observations of the C 

line 503 

149. "Motion forms "and "Lozenges** 523 

150. Arrangement for throwing image 

of light-source on the slit . . 537 

151. Long and short lines of zinc and 

cadmium 540 

152. Long and short lines of lead and 

cadmium 541 

153. Plan and section of cup used 

with salts 543 

154. Aluminium cup placed in the 

spark-stand as in use . . . 544 

155. Secchi's types of stellar spectra 558 

1 56. Young's observation of the spec- 

trum of a solar spot between 
C and D 564 



PIG. PAGB 

57. Rayet's, Herschers, and Ten- 

nant*s observations . . . .573 

58. Slit for comparing spectra of 

centre and hmb 576 

59. Path of ray 576 

60. Mr. Hastings' arrangement for 

comparison of spectra . . .577 

61. Secchi's Merz-prism before the 

object-glass in cell .... 582 

62. The cell without the prism . . 582 

63. Diaphragm with annulus used 

by Lockyer and Seabroke . 583 

64. Annulus viewed and brought 
to focus 584 

65. Solar profiles 585 

60. Exlge of chromosphere (billowy) 585 

67. Edge of chromosphere (jagged) 586 

68. Young*8 observation of a pro- 
minence 588 

69. The same 589 

70. Ferrari's observation of a change 
of wave-length 596 

71. Young's first observations of 
alterations of wave-length . . 598 

72. His subsequent observations . 600 

73. His subsequent observations . 600 

74. Prominence seen by D^ D and 

D3 light 602 

175. Diagrams . showing magnetic 

storms 639 



: ERRATA. 

PACK 

137 line 3 from bottom, for incidently, read incidentally. 

155 „ 7 „ for 1812, read 1802. 

265 ,,2 ,, for its, read OVLT, 

299 omit upper horizontal line from diagram. 

376 line 2 from bottom, for radial of, read of radial 

411 „ 12 „ y^TT phenomena, r^u^ phenomenon. 

454 9t 6 from top, y^ solar, roi^ polar. 



PART I. 

A POPULAR ACCOUNT OF ANCIENT AND MODERN 
INQUIRIES INTO THE PHYSICAL CONSTITUTION 
OF THE SUN, WITH SPECIAL REFERENCE TO 
RECENT SPECTROSCOPIC RESEARCHES. 



SOLAR PHYSICS. 



WHAT IS A SUN? 



Ages ago, when time was younger than it is now, and chap. i. 
human curiosity had a whole virgin universe to revel 
in, when the stars were unmapped and space had been 
pierced by no sounding-line, what wonder that man, all 
ignorant of better things, looked upon the Sun — and wor- 
shipped it ? 

Although we know better than this now ; although 
instinct has given place to reason, superstition to science, 
and curiosity to inductive philosophy, the mystery of the 
sun still remains, and we present earth-dwellers wonder 
at its mighty power with even greater reason than the 
men of old. 

For, in fact, modem science, dealing with the whole field 
of the natural, and leaving the j///^matural out of the 
question, as science must ever do, has almost justified, so 
to speak, the instinct of those who added adoration to won- 
derment. The power they felt, we have proved to exist ; 
the world-supporting, life-sustaining influence with which 
they invested the glorious orb of day, we now know 

n 



SOLAR PHYSICS, 



Eirly 



CHAP. I. belongs to it by right ; man has measured its distance, 
has weighed it in the balance, and has glimpsed the forces 
at work on its surface ; and he everywhere finds a stupen- 
dousness which baffles him and far surpasses anything the 
imagination of the early inhabitants of this planet could 
have conceived ; and now that we know so much, rises 
as of old, towering above the mighty inquiries which have 
resulted in our present knowledge, the all-embracing 
question, " What is a Sun ? " 

Passing over the era of Sun-worship, which has left 
its mark in every land in which early man dwelt, let us 
endeavour to learn something as to the notions concern- 
ing the sun which were rife when the Western civilization 
began to dawn. We find as answers to the question 
*/ What is a Sun ? " what we must designate as the wildest 
guesses, although some of them came from some of the 
greatest minds of antiquity. 

Thales, one of the first of the Greek astronomers, whose 
^6'd'^ll(i G^rly life was spent in Egypt, where he deeply studied 
the lore of the priests, had ideas concerning the sun 
which, it will be seen, were in the main sound ; and it is 
curious that with such a good beginning subsequent 
philosophers went so far astray. He held that the sun 
and stars were of an earthy or solid substance, and that 
their light was due to fires fed by aqueous exhalations. 
He also explained correctly both solar and lunar eclipses, 
thereby implying that he thought that the moon shone by 
reflected light. 

The anecdote of Thales falling into a well,^ which is 
related by Plato, with the consequent saying of the Thra- 
cian female slave, that in trying to discover things in 
heaven he overlooked those beneath his feet, is widely 
known, and proves his popular reputation as an astronomer 
and star-gazer. 

' Plut. Plac. Phil. i. 3, as quoted by Sir (». C Lewis in his admirable 
" Historical Survey of the Astronomy of the Ancients," p. 82, to 
which I am indebted for much information recorded in this cnapter. 



Thales 
livAi from 
' } to \ 
B.C. 



WHAT IS A SUNf 



But when we come lower down the stream of time, to 
580 B.C., and seek what was then the prevailing opinion, 
we find Anaximander at the head of a school which ex- 
plained the various solar phenomena by supposing that 
the sun and stars were bodies of condensed air^ containing 
fire which escaped through certain apertures. He further 
imaged the sun as a wheel, the rays, forming the spokes, 
being emitted from the aforesaid apertures. His more de- 
tailed teachings in solar and lunar physics are thus stated 
by Sir G. C. Lewis '} he held that the sun was "of circular 
form, with an opaque annular band on its exterior, the 
circumference of which is twenty-eight times that of the 
earth ; that within this annular band is a fiery central 
portion, equal in size to the earth ; that the movement of 
the sun is due to its opaque ring, and that an eclipse of 
the sun takes place when the central aperture is closed. 
His hypothesis regarding the moon was similar : he sup- 
posed the luminous centre to be seen through a tube, like 
the mouth of a bellows ; that the eclipses (or phases) are 
cau.sed by the revolutions of the opaque ring. .... 
Hence Anaximander [unlike Thales] held that the moon 
shone by her own light." 

Anaximander's notions, it will be seen, are not over- 
burdened with clearness ; and, indeed, about this time we 
get perhaps the greatest number of wild guesses. Zeno- 
phanes of Colophon taught that the sun was lit and ex- 
tinguished every day, like coals, accounting for eclipses by 
the latter process ; he told, moreover, of one eclipse which 
had lasted a month.* 

The fault, however, found with Anaximander, cannot be 
laid at the door of the next suggestion to be recorded, 
which we owe to Heraclitus of Ephesus. This philosopher 
made himself noteworthy by the following ingenious ideas. 
He held that the sun and moon are bowl-shaped hemi- 
spherical cavities, with a bright side and a dark one. If 
the whole of the bright side of this hemispherical vessel 

* Op. cit. p. 93. ' Op cit. p. 98. 

U 2 



CHAP. I. 

Afiaxi' 

mander^ 

bom 610, 

died 547 

B.C. 



Zenophanes 

of 

Colophon 

flourished 

540 to 500 

B.C. 



Heraclitus 

of Ephesus 

flourishtd 

about 504 



B.C. 



SOLAR PHYSICS. 



CHAP. I. 



Anajca- 

goras of 

Clazo- 

mma^ born 

499, died 

about 430 

B.C. 



Aftaxi' 

metusy 

born 475 

B.C. 



is turned towards us, we see the uneclipsed sun and full 
moon. If the whole of the dark part is turned to us, then 
we see the sun eclipsed, and the moon is then new ; and 
he further showed how, on this simple hypothesis, by 
imagining the bowls to be turned now a little one way 
and now a little the other, it was perfectly easy to account 
for the phases of the moon and the various kinds of eclipses 
of the sun.^ 

The sun, according to this philosopher, was a body of 
compressed fire fed by exhalations ; it was no bigger 
than a man's foot, and brighter than the moon in con- 
sequence of its position in a clearer atmosphere than that 
near the earth. 

We must next refer to Anaxagoras of Clazomenae, who 
was second to none among the Greek philosophers in his 
intense interest in cosmical speculations. He is stated to 
have said that he was born for the contemplation of the 
sun, the moon, and the heavens;'-^ in fact his ideas on 
physical subjects were so advanced that he was accused 
of impiety. It is not a little curious that the charge of 
atheism which was preferred against him should not have 
been made before, for surely the charge of substituting 
mechanical force for the direct agency of the Gods would 
have applied equally to Anaximander or Heraclitus of 
Ephesus. And it is also curious that his doctrine that 
the sun was a mass of ignited stone larger than the Pelo- 
ponnesus, instead of a God who drove his chariot across 
the sky, was one of the main charges against him. It is 
consoling to think that Pericles saved his life at all events 
by his eloquent defence. 

Next, Anaximenes held that the sun, instead of being 
a globe, was flat like a leaf; "that the stars, being im- 
pelled by the resistance of the condensed air, cause the 
solstitial movements of the sun ; that the nature of the 
moon is igneous, and therefore that she shines with her 
own light ; that the earth is a flat trapezium ; and that on 

' Op. cit. p. 96. a Op. cit. p. 103. 



WHAT IS A SUNf 



account of this form it is supported by the air without chap. i. 
sinking. He applied the same doctrine to explain the 
suspension of the sun and moon in space." ^ 

Empedocles, the contemporary of Herodotus, held that EmpedocUs 
there were two suns, one of fire in the lower hemisphere, /^m"^^s 
and another, the upper one, *' reflecting its own light /<'444bc. 
from the fiery air upon the earth," as stated by Sir G. C. 
Lewis. 

It is but fair to remark that all these ideas were held 
to be so wild by some of the best men among the Greeks 
that they rebelled very much against them ; so that we find 
the whole genus of astronomical men soundly castigated 
by Socrates, who thought astronomy was desirable for de- 
termining the day of the month or hour of the night, but 
that to carry it further was waste of valuable time, and 
that "speculators on the universe and on the laws of 
the heavenly bodies were no better than madmen." ^ 

It is not till the time of Eudoxus of Cnidos, the con- 
temporary of Plato, that we find any sustained attempt at P/a/o, 
real investigation ; and so impressed was he of the extreme ^ ^^^ ^^^ 
importance of the sun to the earth, that he is recorded by 
Plutarch^ to have declared that he would willingly suffer 
the fate of Phaethon could he only approach the sun near 
enough to determine its real figure and constitution. 

By the time of Aristotle the right of free thought in Aristotle, 
cosmical matters had been thoroughly established, and we ^^^^^^^^ 
find that philosopher defining astronomy as a science 
founded on observation and calculation, although the idea 
that the sun and stars are living bodies eminently par- 
taking of the divine nature occurs in his writings.* 

From this time, throughout what we may term the pre- 
telescopic age, it would appear that the question of the 
physical constitution of the sun and stars gradually gave 

» Op. cit. p. 06. 2 Op. cit. p. 113 

* Quoted in "Astronomy of the Ancients,*' p. 148. 

* Op. cit. p. 163. 



SOLAR PHYSICS, 



c»iAP. I. way to those which dealt with the various motions of 
each member of the starry host: 

"Cycle on epicycle, orb on orb," 

formed the key-note of a large part of astronomy after 
this time until we come, nearly two thousand years later, 
to Galileo and his contemporaries, when we again find 
ourselves in the midst of a sea of speculation on solar 
matters. 

Needless to say that this was no accidental circum- 
stance. The telescope had been discovered, and with it at 
the first essay the visible universe had been infinitely 
expanded. The period of the invention of the telescope 
is one of the most interesting and momentous in the 
scientific history of the world : in it the golden age of 
astronomy may be said to have dawned. 
iCfpler, We find then living on this planet three immortal men. 
Br^^f and Fi^st Kepler, who by his theory of elliptic motion at one 
Galileo, blow swept away the elaborate work of twenty centuries 
.on cycles, epicycles, and excentric motions.^ Secondly, 
Tycho Brahe, whose admirable observations formed the 
groundwork of Kepler's investigations. Lastly, Galileo, 
who was at home in all branches of science : a man of 
tremendous mind ; who independently, and as it were by 
the way, invented the telescope, and who was not only the 
first man who applied it to celestial objects, but perhaps 
the most illustrious man who ever did so. 

We all know his reward. Like Anaxagoras, he offended 
against current dogmas, and in Christendom or Heathen- 
dom the penalty for that always v/as, nay almost i>, the 
same. 

According to Galileo's own story — and who shall doubt 
It } — he began the telescope's work on the sun in the 

* The words with which he sent his discover)' forth are too admi- 
rable not to be quoted in this place : — " I have finished my book. 
It will be read by the present ape or a future one — I care little which. 
It can well wait for a reader, for has not God waited 6,000 years 
for a contemplator of H is works ? " 



WHAT IS A SL St 

month of October 1610,' being followed by the Jesuit 
Scheiner in April 161 1, according to his own statement, 
and by Fabricius in June of the same year. 

It is pleasant to think that some of the earliest observa- 
tions of the sun were made by a countryman of our own, 
and in the Tower of London. I allude to the eminent 
mathematician Harlot, who observed the spots as early as 
December 161 1, though not as eatly as December 1610, as 
was once thought by De Zach.'' 




Needless again to say that the old-world answer to our 
question " What is a Sun ? " was at once changed. It was no 
longer immaculate ; no longer, as in the thought of Aris- 

' See on the whole question of disputed priorit}- in this matter, 
"GaliWc, sa Vic, ses Ddcouvenes, et ses Travaux." Far le Dr. M.is 
Parchappe. Hachette, t866. P. 92 et seq, 

• Grant's " History of Physical Astronomy," p. '15. 



SOLAR PHYSICS. 



CHAP. I. totle, a material image of the spotless purity of the divine 
mind. Galileo discovered spots on the brightly shining 
orb, and the thrill that ran through the world of Schoolmen 
as a result of this announcement may well be imagined ! 

Galileo's discoveries, thanks to the controversy which 
was raised by the Jesuit Scheiner, who wished to obtain all 
the credit for Galileo's work, have come down to us in full 
detail: for to defend himself, Galileo, in a remarkable 
series of letters to Welser, the chief magistrate of Augs- 
burg, gives us the whole history of his work, which here, 
however, can only be lightly touched upon. 
Galileo's In his first letter, under date May 4, 161 2, he states that 
first litter, ^j^^ spots are real, as bright as the moon, and composed of 
matter not very dense and differently shaded ; that they 
are not permanent, but resemble our clouds ; and that it 
is absurd to think they are planets, which was Scheincr's 
opinion about this time. 
Second In his second letter, dated August 14 in the same year, 
letter. j^g refers to their common movements, and to their limita- 
tion to zones ; states that they lie near the sun, and move 
on it and not above it ; and then announces, from obser- 
vations of them made near the sun*s edge, first, that they 
are deep and of various depths, and secondly, that their 
blackness diminishes near the edge. 
TTiird In his third letter, written on the lOth of December, 1612, 
letter. j^^ demonstrates the sun's rotation, and assigns a period 
not far from that given by modern observation. 

Scheiner, who wrote under the signature of Apelles 
latens post tabulam, to the same Welser, held on his side 
that the spots were similar to Jupiter's moons, or pro- 
bably to the strange things which Galileo had then re- 
cently discovered round Saturn, or that they might be 
comets. He held that it was impossible they could be 
on the sun itself, and imagined some to be as far from the 
sun as the Moon, Venus, or Mercury (on the Ptolemaic 
system). At the same time he pointed out that they 
are //////, to account for their oval appearance near the 



WHAT IS A SUA'f 



limb, adding that they are not fixed stars, although they 
arc as dense as the moon. 

It is not necessary in this brief sketch to do more than 
refer to the detailed work of Scheiner recorded in his 
" Rosa Ursina," or to the subsequent observations of 
Fabricius and Hcvelius.' It is clear that the answer to 
our question was now much closer; indeed we have the 
spots and no longer the sun itself, due to evaporations 
and exhalations, the luminary itself being described as a 
" liquor igiiciis . . . quasi vastissimum luiiiimtm pelagiis et 
mare igncus, quod siios /label abyssos, occullos mmtus vara- 
gines atqite vortices." 

Ournext step carries us to 1774; for in De la Hire's 
memoir presented to the Paris Academy in 1704. there 
is nothing that calls for remark. 



^Bf ^P P 







In 1774, Dr. Wilson of Glasgow communicated a paper 
to the Royal Society,* demonstrating that the spots were 

' The reader may wilh profit cnnsull Cram's " History of Physical 
■■ 313^/1.-?. • Phil. Tratis. 1744. 



lO 



SOLAR PHYSICS. 



CHAP. I. 

IViisan's 
work. 



Dela 
Lande. 



Sir \Vm. 

HcrscheVs 

%iH>rk, 



cavities in a luminous envelope surrounding the sun, which, 
according to him, was a dark globe. His observations were 
made on the great sun-spot of 1769. The reasoning on 
which he based his idea of the cavernous nature of the 
spots will easily be gathered from an inspection of the 
woodcut, Fig. 2 : it will be seen that it depends upon the 
different appearances put on by the same spot as seen in 
the centre of the disc and near the sun's limb. In the 
case of a spot, supposed round, seen in the first position, 
it is clear that we shall have a round, black shade in 
the centre, equally surrounded by a half-tone ; while when 
the spot is near the limb it is equally clear that on this 
hypothesis the central black shade will be almost entirely 
hidden, and the half-tone quite hidden on one side and 
largely developed on the other. Now this is exactly what 
is observed. 

To this paper of Wilson's, which is now acknowledged 
to be one of the most important contributions to our 
knowledge of the sun, in 1776 De la Lande replied in a 
paper presented to the Paris Academy. He does not agree 
with Wilson, although he gives up the idea which he had 
formerly held, that the spots were masses of scorix, and 
then he adds: — "J'ai done pens6 que les taches ^taieiit 
plut6t les Eminences d'un noyau solide, decouvertes et re- 
couvertes alternativement par le flux et le reflux de la 
matiere ign^^c ou elles sont presque toujours plongdcs . • . 
les ncbulosit^s [faculai] qui environnent les taches et qui 
ressemblent k des bancs de sable prdsentent I'id^e d*un ba^ 
fond qu'on aper9oit k Tendroit ou la matiere fluide a 
moins de profondeur." 

We must last of all in this chapter deal with Sir William 
Herschers^ answer to the question at the head of it. 

Aided by telescopes of his own manufacture, he doubtless 
was the first man on our planet to sec the sun in all its 
beauty and detail, as it is now seen with even small instru- 
ments of modern make. 

» Phil. Trans. 1795. 



WHAT IS A SU^f 



Herechel's fixed idea, evidently one gained from his first 
observations, was tliat the darkness of a spot was an 
indication of a cool habitable globe under the shining enve- 
lope of the sun ! and long before this theory is distinctly 
enunciated, some of the phenomena of the penumbrE are 
explained by the existence of " mountainous countries." 
though he acknowledges that such mountains must be at 
least 600 miles high. 




Although it is possible that we may here trace the influ- 
ence of the opinion of De la Lande, it is clear that he is 
a firm believer in Wilson's hypothesis, for the old nomcn- 



SOLAR PHYSICS. 



clature, which will be fully stated in the next chapter. 
is altered in Wilson's sense. Instead of "spots" we 
have "openings," while spots without nuclei are termed 
"shallows," connected facula; "ridges," and separate 
faculje "nodules:" the delicate mottlings of the general 
surface are also called " corrugations." 
, As a result of his labours, Herschel modified Wilson's 
■ hypothesis in this wise. Round Wilsons dark globe, which 
he considered inhabited, instead of one envelope he placed 
t\vo, the exterior one consisting of empjreal, luminous or 
phosphoric clouds residing in the solar atmosphere ; the 




interior one cloudy and opaque like our own, and highly 
reflective on its upper surface, so that the light and heat of 
the upper envelope were tempered, so to speak, to the solar 
inhabitants. He held that the solar atmosphere reaches 
to a great height, and is of great density, and that there 
is a clear space between the lower or "planetary" cloud 
envelope and the body of the sun. 

This, then, was Herschel s idea : what the modern one is 
will be gathered from the following chapters. 



ON THE TELESCOPIC APPEARANCE OF 
THE SUN. 



It is now more than two centuries and a half since the ■ 
first telescope was turned upon the sun, and longer still 
»nce Fabricius, unmindful of Appian's recommendation of 
the use of coloured glass as a shield to the eyes, was wont 




— Eariy SuD-wnrk. Schcir 



to watch the wonderful spots crossing the solar disc. This 
he accomplished by allowing the sun's rays to enter through 
a small aperture into a dark room, and projecting its image 



14 SOLAR PHYSICS. 



CHAP. II. on a piece of white paper, or, again, by viewing it 
directly when near the horizon. This latter method — the 
one pursued by Galileo in his observations — was improved 
upon by Scheiner, who placed a coloured glass between 
the object- and eye-glasses, and still more by Sarde^ who 
in 1620 placed it between the eye-glass and the eye. 

Here, then, we find the first employers of "optick tube " 
manfully battling against the obstacle which, even down 
to our own time, has proved all but insurmountable in 
observations of the sun — we mean his brightness, com- 
pared with which the lime-light pales its ineffectual fires, 
and appears even black when projected on his disc. 
Methods of Among the attempts which have been made in later 
observation, ^j^^g ^q observe the sun satisfactorily, we are no longer 

limited to the employment of Sir John Henschel's glass 
speculum, or the diagonal eye-piece, both allowing nearly 
all the heat, and a very large percentage of light, to pass 
through them, and sending the enfeebled, but even still not 
sufficiently enfeebled, beam by reflection to the eye. But 
the light may be deadened by passage through a silver 
film on the object-glass, or by polarization more or less 
total in the eye-piece. It is perhaps to be regretted that 
the Dawes' solar eye-piece — the point of this latter instru- 
ment being, that the quantity of light admitted to the 
eye is reduced by observing only an excessively small 
portion of the solar disc — is falling into disuse. There is 
still another method of research which possesses many ad- 
vantages, although it is a direct method, and as such may 
fuse or break the eye-piece, and give a di.storted image ; 
Use of we allude to the use of a screen — introduced, as we have 
soZTJsIt' s^^"» t)y Fdbricius — which has been employed so suc- 
vdtions. cessfully by Messrs. Carrington, Howlett, and others, the 
screen being of any material — plaster of Paris is the 
best — and being placed at a varying distance from the 
eye-piece, according to the magnifying power required. 
Lastly to be recorded here — we shall have, further on, to 
return to its splendid results — is the art of solar photo- 



TELESCOPIC APPEARANCE OF THE SUN. 



CHAP. II. 



Afore 
ivorkers 
wanted. 



graphy, which in the hands of Mr. De la Rue and Mr. 
Rutherfurd has shown us how inevitably it will some day 
supersede eye-observations of the sun. 

Thanks to these modern methods of research, and the 
recent increase in the number of powerful telescopes brought 
to bear upon our luminary, we have already reaped a rich 
harvest of facts relating to the general surface, the spots, and 
the faculae, which are gradually leading our philosophers 
to some more definite notion of the phenomena which 
these appearances indicate — although, alas ! our telescopes 
are still all insufficient to translate to us all the agencies 
at work, and all the action going on in that wonderful 
globe some ninety millions of miles away. One thing: 
we want more workers. Many of our best observers are 
busy men, who more often see the sun through a glass 
darkly in their places of business, than by means of their 
instruments : and, again, we want more powerful telescopes^ 
and we want these telescopes high above the lower strata 
of our atmosphere. Mr. Howlett's work was, we believe, 
all done with a 4-inch glass — would it had been an 
8-inch! We must not, however, forget that much sun- 
work relates to the position of the spots, and not to their 
physical features. Kew and Ely formerly took spot-maps, 
so to speak, day by day by means of photography ; but 
the sun-pictures obtained were small : at the present The photo- 
moment not only are we nowhere chemically registering ^''^^^// 
spots to the noble scale of a yard to the sun's diameter, stopped. 
which Mr. De la Rue and Mr. Rutherfurd have shown us 
to be practicable, but to our disgrace the photographic 
record has stopped altogether. 



Before we allude to the more recent discoveries in solar 
physics, it will be well to describe as briefly as possible 
the actual general appearance presented by the sun in a 
powerful telescope ; always remembering that our mighty 
luminary is some 91,000,000 miles removed, that its diam- 
eter is 100 times that of our earth, and that the chasms 



SOLAR PHYSICS. 



. we call sun-spots are sometimes large enough to swallow 
us up, and half-a-dozen of our sister planets besides ; 
while if we employ the finest telescope, under the most 
favourable atmospheric conditions, we are only enabled 
observe the various phenomena as we should do with the 
naked eye at a distance of 180,000 miles. 

The first things which strike us on the sun's surface, 
when we look at it with a powerful telescope, are the 
spots, which are not scattered all over the disc, but are 
g'enerally limited to those parts of it a little above and 




below the sun's equator. The spots float, as it were, on the 
bright general surface of the sun, called the p^losp/iere. 

Spots generally exhibit three shades of darkness, and 
float as it were in the bright surface or photosphere, the 
darkness increasing from the general surface till the apparent 
centre of the spot is reached. We have first the penumbra, 
then the umbra, then the nucleus. But sometimes the 
darker portions are excentric, and very irregular in outline. 

Observations of the umbra and penumbra, with powerful 
instruments, have revealed to us the fact that change is 
going on incessantly in the region of the spots. Some- 
times changes arc noticed, after the lapse of an hour even : 




TELESCOPIC APPEARANCE OF THE SUN. 



here a portion of the penumbra is seen setting sail across 
the umbra; here a portion of the umbra is melting from 




llic Sun't S\iC, OcL. iih (fonthancncd vitw). 1. Ocl. lolh. 3. Oct. iitb : ccntnl view. 

Ight ; here, again, an evident change of position and 
Sirection in masses which retain their form. In some spots 



SOLAR PHYSICS. 

evidences of cyclonic action are very obvious (Fig. 6). 
The enormous changes, extending over tens of thousands 
of square miles of the sun's surface, which took place in 
the great sun-spot of 1865, are shown in the preceding 
engraving. 

We next come to the brighter portions of the general sur- 
face, which are well seen near the edge of the solar disc, 
and especially about spots approaching the edge; in these 
positions it is quite easy, even with a small telescope, to 
discern bright streaks of diversified form, quite distinct in 
outline, and either entirely separate or uniting in various 
ways into ridges and network. These appearances, which 
have been termed /aailcE, are the most brilliant parts of the 
sun. Where, near the edge, the spots become invisible, 
undulated shining ridges still indicate their place — being 
more remarkable thereabout than elsewhere, though every- 
where traceable in good observing weather. Faculx may 
be of all magnitudes, from hardly visible, softly-gleaming. 




MifHifuJe narrow tracts i.cxx) miles long, to continuous complicated 
effaiuu. g^j heapy ridges 40,000 miles and more in length, and 
1,000 to 4,000 miles broad. Ridges of this kind often sur- 
round a spot, and hence appear the more conspicuous ; such 
ridges are shown in Fig. 8, but sometimes there appears 
a very broad white platform round the spot, and from 
this the white crumpled ridges pass in various directions. 



TELESCOPIC APPEARANCE OF THE SUN. 19 

So much for the more salient phenomena of the sun's chap. ii. 
surface, which we can study with our telescopes. There 
is much more, however, to be inquired into. We may thesofar 
begin by saying, that the whole surface of the sun, except ^rface, 
those portions occupied by the spots, is coarsely mottled ; 
in a large instrument, it is seen that the surface is prin- 
cipally made up of luminous masses — described by Sir 
William Herschel as corrugations — and small points of 
unequal light, imjperfectly separated from each other by 
rows of minute dark dots, called pores, the intervals between 
them being extremely small, and occupied by a substance 
decidedly less luminous than the general surface. The 
luminous masses present to different observers many varie- 
ties of irregular forms, and hence it was that a few years 
ago we had the famous "Willow-leaf Controversy," now 
at last put to rest by the more decided outcome of recent 
discoveries. 

The originator of this controversy was Mr. Nasmyth, Afr, 
who announced the discovery of willow-leaved things — ^^^i^^/ 
which were afterwards suggested to be solar organisms I — leaves:' 
nay, the definition ran still closer: they were solar dia- 
tomacecB, covering, like so many scales, the whole surface 
of the sun. Aided by a refractor (of 8 inches' aperture, 
we believe) by Cooke, he found that — it is Sir John Herschel 
who narrates his discovery, and clothes it in his own beau- 
tiful language^ — "The bright surface of the sun con- 
sists of separate, insulated, individual objects or things, 
all nearly or exactly of one certain definite size and 
shape, which is more like that of a willow-leaf as he de- 
scribes them, than anything else. These leaves, or scales, 
are not arranged in any order (as those on a butterfly's 
wings are), but lie crossing one another in all directions, 
like what are called spills in the game of spillikins, 
except at the border of a spot, when they point for 
the most part inwards toward the middle of the spot, 
presenting much the sort of appearance that the small 

1 Good Words, April 1863, p. 282. 

(• 2 



20 



SOLAR PHYSICS. 



CHAP. II. leaves of some water-plants or sea-weed do at the edge 
of a deep hole of clear water. The exceedingly definite 
shape of these objects, their exact similarity one to an- 
other, and the way in which they lie across and athwart 
each other (except where they form a sort of bridge 
across a spot, in which case they seem to affect a 
common direction, that, namely, of the bridge itself) — all 
these characters seem quite repugnant to the notion of 
their being of a vaporous, a cloudy, or a fluid nature. 
Nothing remains but to consider them as separate and 
independent sheets, flakes, or scales, having some sort 

of solidity These wonderful objects have been .seen 

by others as well as Mr. Nasmyth, so that there is no 
room to doubt of their reality. To be seen at all, how- 
ever, even with the highest magnifying powers our tele- 
scopes will bear when applied to the sun, they can hardly 
be less than a thousand miles in length and two or three 
hundred in breadth." 

When this discovery was announced, an observer of the 
highest eminence, the Rev. W. R. Dawes, at once denied 
the reality of these appearances ; and a Paper by him, 
read at a meeting of the Astronomical Society, and the 
discussion upon it, gave rise to quite an excitement in 
the astronomical world. Mr. Dawes in his paper^ re- 
marked : — 

" When carefully scrutinized with large af)ertures and 
high powers, under suitable atmospheric circumstances," 
solar phenomena "are so wonderfully different in their 
appearance from those presented by the diminished aper- 
tures formerly and necessarily in use, that it would not be 
very surprising if some observers, unaware of what had 
been previously seen and described, should imagine that 
the phenomena revealed by their newly acquired and 
powerful telescopes were really new discoveries ; and this is 
what there is good reason to believe has in some instances 
actually occurred. Such a mistake may also be more 

* Monthly Notices, R. A. S., 1863, vol. xxiv. p. 33. 



Tht Rn\ 

W,R. 

Dawei 

ohserr'a- 

iiont. 



TELESCOPIC APPEARANCE OF THE SUN. 21 

likely to be made when a new name has been applied by chap. n. 
some recent observer to an appearance long familiar to ~ ~~ 
others, though previously unnamed. A name, no doubt, obsena- 
has the advantage of affording a convenient handle ^'^''^• 
whereby to grasp the thing described ; but, unless it 
is very correct and appropriate, it conveys an erroneous 
impression of the appearance to which it is applied, and 
may become far more injurious than advantageous. It 
seems to me, therefore, to be desirable to direct attention 
to what has been long ago observed and described ; also 
to put on record some results of a pretty constant scru- 
tiny of solar phenomena with powerful and excellent 
telescopes during the last twelve or fifteen years. 

"The mottled appearance of the solar surface requires Motthdap- 
no very large amount of optical power to render it visible, th^^n's 
I have often observed it with a good refractor of only 2\ surface. 
inches' aperture and a power of 60. Examined with a 
large aperture, such as 6 or 8 inches, it becomes evident 
that the surface is principally made up of luminous masses 
imperfectly separated from each other by rows of minute 
dark dots — the intervals between these dots being ex- 
tremely small and occupied by a substance decidedly less 
luminous than the general surface. This gives the im- 
pression of a division between the luminous masses, espe- 
cially with a comparatively low power, which, however, 
when best seen with high power, is found to be never 
complete. The masses thus incompletely separated are 
of almost every variety of irregular form, the rarest of all, 
perhaps, being that which is conveyed to my mind by Mr. 
Nasmyth's appellation of * willow- leaves,' i.e. long, narrow, 
and pointed. Indeed, the only situation in which I have 
usually noticed them to assume anything like such a shape 
is in the immediate vicinity of considerable spots, on their 
penumbra, frequently projecting beyond it irregularly for 
a small di.stance on to the umbra, — an appearance with 
respect to which, in my ' Description of a new Solar Eye- 
piece' which I read before this Society in April 1852 (see 



22 SOLAR PHYSICS, 



CHAP. II. ' Memoirs/ vol. xxi. p. i6i), I employed the following 
expression : — * The interior edge of the penumbra fre- 
quently appears extremely jagged ; the bright ridges on its 
surface, which are directed nearly towards the centre of 
the spot, being seen projected to irregular distances on to 
the cloudy stratum (or umbra), and looking much like a 
piece of coarse thatching with straw, the edge of which has 
been left untrimmed/ After nearly twelve years of care- 
ful observation of the same phenomena, I do not think 
I could improve upon this description." 

Mr. Dawes then refers to Sir John Herschers description 
in his " Outlines of Astronomy," art. 307, where he 
says : — " The part of the sun's disc not occupied by spots 
is far from uniformly bright. Its ground is finely mottled 
with an appearance of minute dark dots or pores, which, 
when attentively watched, are found to be in a constant 
state of change. There is nothing which represents so 
faithfully this appearance as the slow subsidence of 
some flocculent chemical precipitates in a transparent fluid, 
when viewed perpendicularly from above." 

Mr. Dawes endorses this description — with the exception 

of the constant change of the pores, which he denies, 

ascribing the appearance to tremors in the atmosphere — 

MotioH of and goes on : — " A striking exception, however, to this 

KT^ifhf- comparative quietude is found in the immediate vicinity 

sphere of spots which are either rapidly enlarging or closing. It 

*^7pou^ is under these circumstances especially that the luminous 

masses are found to become more elongated. This is also 

more remarkably the case when they are preparing for a 

rush across a chasm, and thus forming those luminous 

bridges which so often intersect considerable spots. The 

point from which such a rush is about to be made is often 

indicated by a general crowding together towards that 

place, and a general inclination of the longer axis of each 

of the elongated masses in that direction, which might, as I 

imagine, be well exemplified by such chemical precipitates 

as Sir John Herschel alludes to, if they were about to flow 



TELESCOPIC APPEARANCE OF THE SUN. 



23 



through a narrow spout or opening in the vessel containing 
them." 

The question was soon narrowqd to this : Nasmyth de- 
clared for willow-leaves all over the sun ; Dawes declared 
for something like them, which he calls " bits of straw " or 
" thatch," which are to be seen only round the spots where 
they are formed from the general flocculent covering of 
the sun, by a drawing out of it, as it were, by some action 
in their neighbourhood. Sir John Herschel still held to 
his old definition — "a slow precipitation of flocculent 
matter." It was not long before it was found out that the 
willow-leaves were "wanted." Mr. Pritchard told us, on 
the authority of Sir John Herschel, that "the advancing 
state of our philosophy required the willow-leaves: and 
they came just in the nick of time." Sir John Herschel 
remarks, in the article we have before alluded to : " These 
flakes, be they what they may, and whatever may be 
said about the dashing of meteoric stones into the sun's 
atmosphere, &c., are evidently the immediate sources of 
the solar light and heat, by whatever mechanism or what- 
ever processes they may be allowed to develop and, as it 
were, elaborate these elements from the bosom of the non- 
luminous fluid in which they appear to float. Looked at 
in this point of view, we cannot refuse to regard them 
as organisms of some peculiar and amazing kind ; and, 
though it would be too daring to speak of such organiza- 
tion as partaking of the nature of life, yet we do know 
that vital action is competent to develop both heat, light 
and electricity." 

The controversy which began in 1863 was still going 
on in 1865, with the distinct advantage that a rapidly in- 
creasing share of attention was given to solar phenomena. 
In France, M. Chacornac made important observations, 
while in England many good observers communicated 
papers to the Astronomical Society. 

On the 2nd and 3rd of April there was a fine spot 
on the sun, which I was enabled to study under admi- 



cHAP. 11. 



Opinions of 
DaweSy 

Nasmyth^ 

and 
Herschd* 



Theflakes 
or willow- 
leaves the 
sources 0/ 
heat and 
light. 



M. Chacor- 

nac^s 

observa- 

iions in 

Prance. 



24 



SOLAR PHYSICS. 



CHAP. II. 



Sfcchi 

beiuves the 

photosphere 

cloudy. 



Ilerschel 
believes the 

willaut- 

leaves to be 

solid 

matter. 



rable atmospheric conditions, with the result that my own 
mind was soon made up on the controversy, and that 
important evidence as to a downrush into a spot was 
obtained. 

My communication to the Astronomical Society ran as 
follows : — 

" On the same leaf of the last number of the Monthly 
Notices^ we have two opinions on the nature of the matter 
of which the sun*s photosphere is composed, coming from 
two men whom we must all respect, — Father Secchi and 
Sir John Herschel. Father Secchi writes as follows : — 

" ' I cannot divest myself of the opinion that the photo- 
sphere is really made up of clouds, and that the luminous 
stratum is actually constituted like our clouds, the only 
difference being that the clouds on the earth are of watery 
drops or crystals, and in the sun they are of some other 
substance.* 2 

"Then we turn over the leaf, and we get Sir John 
Herschers opinion : — 

"*....! suppose Mr. Nasmyth will have to give up 
the regularity of shape and equality of size of his " willow- 
leaves" — cloudsy in the ordinary sense of the word, I do 

not think they are I believe them to be pertpia- 

nently solid mattery having that sort of fibrous or fila- 
mentous structure which fits them when juxtaposed by 
drifting about and jostling one against another to collect 
in flocks, as flue does in a room.'^ 

'* Now it is quite impossible that both these opinions can 
be right ; and hence it is that observations such as those 
wc have just listened to are most valuable at the present 
time. I am glad, therefore, to have the opportunity of 
bringing some very imperfect observations of my own 
before the notice of the Society, and I do so simply with 
a view of showing that the matter need not long remain in 
doubt. I must not, however, take any credit to myself as 

i Monthly Notices, R. A. S., 1865, vol. xxv. p. 236. 
Ibid., vol. xxv. p. 151. • Ibid., vol. xxv. p. 152. 



TELESCOPIC APPEARANCE OF THE SUN. 



3S 



1 



being the first to observe what I am about to describe, chap, w. 

because, as early as 1853, I believe, M. Chacomac made a ^ 

similar observation. In a recently published letter' M. Chaiemac 

Chacomac remarks : — r™e*^B- 

'" Having had the occasion formerly to observe small j/An-if 

pieces of silver bathed in borax, melting under the infiu- ""fiZJ" 




ence of the blowpipe, I have always in my descriptions 
compared the "crystals" of the photospheric matter to 
this silver solder in a state of fusion. Of the same opinion 
as Mr. Dawes, I hold that "straws" are the objects which 



Reader,]in. ?, 1865. 




26 SOLAR PHYSICS. 



CHAP. II. give the best idea of the appearance of the objects of 
Chacor- ^^^^^ ^^^ whole solar photosphere consists. On the other 

naesobser- hand, I do not find that in the many notes, and espe- 
vatwm. cially in those of Father Secchi, containing observations 
of this nature, mention is made of any important phe- 
nomena presented by these incandescent "willow-leaves" 
or "rice-grains." As it is inherent in the nature of this 
matter, I will endeavour, as concisely as possible, to state 
in what it consists. If we observe a "crystal" of photo- 
spheric matter which is completely isolated and projected 
on a dark portion of a spot, for instance, it will be seen 
that for a certain time it diminishes in volume and becomes 
spotted over with small dark pores ; that it is subdivided 
into numerous crystals, as if the photospheric matter were 
being volatilized, or as if there were a re-absorption going 
on, absolutely in the same way as crystals of sugar melt 
under a current of steam.* 

" I also have been able to see this diminishing of volume ; 
but I have not been able to see the dark pores. From 
Mr. Fletcher's paper I gather that he has seen them. The 
spot to which I wish to draw particular attention was 
observed on the 2nd and 3rd April. The observation of 
the 3rd was very cursory indeed ; in fact, my attention 
was drawn entirely away from the spot by the general 
appearance of the sun. Of this more presently. On the 
2nd of April the spot was extremely remarkable. It was 
a spot of the normal character, by no means cyclonic, but 
with a tongue of what appeared to be a portion of facula 
— I judge so from its extreme brightness — stretching half 

Changes in way into the spot, as it were. When the observation 

brilliancy commenced, about half-past eleven, the tongue of facula 
was extremely brilliant; by the time I left the telescope, 
about one o'clock, that same tongue of facula seemed to 
be less brilliant than any portion of the penumbra. At 
the same time it seemed to me to be * giving out,' as it 
were, at its end, and a portion of the umbra between it 
and the penumbra appeared to be veiled with a stratus 



TELESCOPIC APPEARANCE OF THE SVN. 



cloud evolved out of it. After a time lai^e, very dim chap. ». 
' willow-leaves ' seemed to be forming (condensing) on the 
following portion of the cloudy mass. So that at first you 
got a very brilliant mass of what appeared to be facula 
gradually melting away into umbra, and then the umbra 
condensing into ' willow- leaves.' I know that, in the case 
of an observation of this extremely delicate kind, it would 

be rash to say that one absolutely puts the right reading 
on what one saw ; but I certainly feel convinced in my own 
mind that I saw a bright mass of facula— I say facula on Thefatula 
account of its brightness, but I do not think it was actually '^'J".' 
above the level of the photosphere — gradually expand a cloud. 
itself into a cloud, and then the cloud became edged with 
'willow-leaves,' which increased in number. At one time 
I saw two, clearly and distinctly. Some little time after- 
wards I saw five or six. I imagined I had made a mis- 
take ; but I do not now think it possible, for the atmo- 
spheric conditions were extremely good ; and I had well 
scrutinized the region before. I was also enabled to watch 
three or four cloud-masses on the inner edge of the 
penumbra detach themselves from it at different points, 
and traverse the umbra towards the centre of the spot. 
Now, in a cyclonic spot this is easily intelligible, but it is 
difhcult to understand two currents opposed to each other, 
or at right angles to each other; the one carrying the 
cloud-masses across the spot, — say from right to left, — 
another carrying them up or down. This, however, is what 
I distinctly saw. I hope that observers who have better 



28 SOLAR PHYSICS. 

LiiAp, 11. eyes and better instruments than mine will bring their 
attention to this subject 

" It has been taken for granted at present by MM. Faye/ 
Chacornac, and others, that there is a downward current 
into a spot, and my observations, I think, show this down- 
rush. It will be of great importance if the facts — first, that 
there is a down-rush ; second, that this is accompanied 
by the melting of the cloud masses carried down— can 
be estabHshed. 
Change of "I also saw the ' willow-leaves,' or ' rice-grains,' in one 
th"^ "'^ region of the penumbra change the direction of their longer 
teava. axes in about three-quarters of an hour with r^^rd to the 




centre of the spot — in fact they turned round bodily through 
a considerable angle. Others projected on the umbra 
gradually melted away out of sight. One willow-leaf I 
distinctly observed to set sail, as it were, over the umbra, 
and it had travelled a considerable distance when I parted 
company with it. I believe I saw another condense — the 
small one in the right-hand comer of the drawing (Fig. 9). 
" These were some points of interest observed on April 2. 
Early on the following day the definition was, if possible, 
very much more fine. 400 was a power easily used on the 
2nd, and on the morning of the 3rd, the sun bore my 
highest power — 690 — an eye-piece which one uses but 

> So runs my paper in the Monthly Notices, but I fear I have done 
M. Faye an injustice, as ever since, until January 1873, when I add this 
note, he has advocated an upnish in spou. 



TELESCOPIC APPEARANCE OF THE SUN. 



rarely to advantage, with my aperture (61 inches Cooke), cbup. i 
With this the surface of the sun was distinctly seen, and, 
thanks to the London fog, which cut off the heat-rays and 
allowed the object-glass to do its utmost in the way of 
definition, I did not require any dark glass. Drawing No. 9, 
which was hastily sketched on the 2nd of April, shows the 
great difference in sliape and appearance "f the 'things' 




BO uQ the KctioL vuiux. 

(whatever they may be) on the general surface of the 
sun, in contradistinction to the 'willow-leaf — or 'rice- 
grain" — things visible in the penumbra. This great differ- 
ence was more than ever established in my mind by what 
I was enabled to see on the 3rd. 

" I am well aware that my opinion on a matter like this 
is not of great value ; but still it may be worth while to 

' These words are used merely wilh reference to the shapes they in- 
dicate; the second edition of Mr. Nasmyth's '■Willow-Leaves" includes 
the stumpy rice-grain. The difference between the actual si(e of the 
rice-grain and Mr. Nasmyth's estimate of the size of the willow-leaves 
is not in question.— J. N.' I- (Note added r86s.) 




f 



SOLAR PHySICS. 

point out that I did see "willow-leaves" or 'rice-grains' 
pretty regularly shaped in the penumbra, and therefore 
might be expected to have seen them on the general surface 
of the sun had they been there. But I certainly saw 
nothing of the kind on the general surface of the sun. It 
would seem indeed that there is a running down of the 
shape ; for whereas on the sun the thing in debate is, 
according to my observation, a confused sort of circular 
mass, you get in the penumbra near the edge of the photo- 
sphere sometimes pointed, sonietiiiics rounded, sometimes 




truncated cloud masses, with a sharpened portion towards 

the umbra, and a very blunt portion towards the genera! 

solar surface. But if you observe similar masses which 

Difftrmi have traversed the penumbra, you find generally that they 

appearance ^^^ pointed at both ends, the point being sometimes rounded, 

aftir tkty sometimcs truncated. Of course it would be extremely 

'"Znumbm '''^''^""^ — indeed it is an observation one can scarcely even 

hope to make— to watch a portion of photosphcric matter 

coming in with a rush, as it sometimes does into the 

penumbra, then travelling through and changing its shape. 



TELESCOPIC APPEARANCE OF THE SUN. 31 



and then finally setting sail and melting ; but I venture to chap. 11. 
think that, if that can be done, it will possibly be found 
that the confused circular mass which is seen on the general 
surface of the sun will be gradually drawn out in its 
journeying towards the umbra ; and if you can get it to 
traverse over the umbra, you will find that it will look as 
much as possible like a willow-leaf" 

Since this was communicated to the Astronomical Society 
Mr. Huggins has given his verdict very much to the same 
effect. 

Now it has been satisfactorily proved, notably by a 
beautiful stereoscopic combination of them, suggested by 
Mr. De la Rue, that the facula^ are higher than the general 
surface of the sun ; that is, where the clouds are highest 
they appear brightest — we see faculcB — because they extend Whywesre 
high into the absorbing atmosphere ; we know that on the ^^^ ^' 
bright surface of the sun rests an absorptive atmosphere, 
because the luminosity is remarkably less near the border, 
indicating that there is a greater thickness of atmosphere 
there which the light has to pass through before it reaches 
our eye. This point will be returned to in the sequel. 
The more minute features — the granules — are most pro- 
bably the dome-like tops of smaller cloud masses, bright 
for the same reason that the faculae arc bright, but to a 
less degree; the fact also that the granules lengthen out 
as they approach the umbra of a spot is similar to the 
effect observed in the clouds in our own sky lengthened 
out when they are drawn into a current ; while the admir- 
able drawings by Secchi reproduced in the preceding pages 
show how plastic the photospheric matter was, to be thus 
torn and contorted. 

But we shall see as we go on what a flood of light has 
been thrown on all these matters during the last few 
vears. 



MR. CARRINGTON'S RESEARCHES ON 

SOLAR SPOTS. 



cjiAi*. III. Spots on the Sun! We can little realize nowadays all 
the hardihood required to make that assertion in Galileo's 
and Scheiner*s time. Spots on that body which Aristotle 
the great master had declared to be the type of everything 
immutable and incorruptible ; maculae on the immaculate ! 
spots on the last stronghold of the spotless ! What wonder 
that even down to our time all the horror set agoing by 
that daring statement has not yet left off vibrating. 

It is now more than 200 years ago since Scheiner, one 
of the first employers of the astronomical telescope, pub- 
lished his great book on the sun, in which these matters 
were first laboriously investigated. Since his time, espe- 
cially in later years, many observers, and among them 
Schwabe, Wolf, Peters, and Laugier, have continued the 
work ; but the wonder and astonishment which they call 
forth are not yet one whit diminished to such men as 
Herschel, and Helmholtz, and Thomson ; nor are they 
the least part of that seemingly invincible mystery which 
surrounds the glorious sun, whose mighty power at last 
seems dawning upon us terricolce, 
Nnvtopts Newton, in his times, was content to ask, "Are not the 
qtt^fy, 5u,^ ^^^ fixed stars great earths vehemently hot } " and 
some 200 years later, in our own, Mr. Carrington is still 
driven to the question, with which we started this book> 



MR. CARRINGTON'S RESEARCHES, 33 



" What is a Sun ? " Now this question is a generic one, chap. hi. 
embracing an infinitude of specific ones of more or less ~ 
importance. Thus, for instance, we want to know some- 
thing of its orbit-sustaining powers, and of the origin of 
these powers ; looking at it, as a " great earth," we want to 
know when it will be as cool as ours is — as a star, if it 
be a variable one either in light or colour. Looking at it, 
again, as a sun, we want to know all its conditions, the 
secrets of its light and heat, of solar physics generally, 
and of the aforesaid spots, which, like straws on a stream, 
tell of the wondrous forces at work. And it is to learn 
something of these spots that Mr. Carrington has been Carring- 
content to observe the sun every fine day for some seven ''^'«*^* 
years and a half, and to deduce the exact position of the 
spots observed. This he has done with a very definite 
object in view, and one which necessitated a forsaking of 
apparently all the most interesting kind of work connected 
with their telescopic appearance. He writes: — "The dis- 
tribution of radiative power, the position of the thermal 
equator, the numerical amount of- illuminating power and 
its possible variations, the estimation even of the degree of 
energy exhibited in the production of spots, and many 
other features, were consciously left aside, and the subject 
before my mind reduced pretty much to tracing regularity 
in the distribution in the maculae, detecting the true period 
of rotation of the body of the sun, and the determination 
of the systematic movements or currents of the surface, 
if such exist, in any definable manner." 

The volume in which these researches appear, though a 
big book, is not the biggest we possess on this subject ; but 
De Lambre tells us that the biggest — Scheiner's " Rosa 
Ursina" — should have rather consisted of 50 than 784 
pages: so Mr. Carrington may take comfort. 

Mr. Carrington,^ distancing all previous inquirers in the 

^ " Obser\'ations of the Spots on the Sun, from November 1853, to 
.March 24, 186 1, made at Redhill." By R. C. Carrington, F.R.S. 
Illustrated by 166 plates. Williams and Norgatc. 

n 



34 



SOLAR PHYSICS, 



Pheno- 
mena of 
earth^s 
rotation* 



CHAP. III. perfection of his instrumental means, the methods of 
reduction employed, and the time he has given to the 
subject, presents us with values of the sun's elements, and 
of the time of its rotation, which it will be very difficult for 
future astronomers to improve upon. 

In order that we may best communicate what he has 
done, let us suppose a boundless ocean in which both 
earth and sun are half immersed. This will represent to 
us the plane of the ecliptic, or earth-plane. Let us further 
suppose both earth and sun to be rotating on their axes 
in a certain period of time, the axes being either upright 
or tipped down — ue, inclined — to a certain extent in a cer- 
tain direction. 

Now, in the case of the earth we know exactly all these 
particular's. We know that our day results from, and is an 
exact measure of, our rotation ; that our seasons are caused 
by a certain tipping down of the axis, and that the pole- 
star marks for us, with sufficient accuracy for our present pur- 
pose, the exact amount and direction of this tipping down. 
The annexed woodcuts (Figs. 15 — 18) will give a good 
idea of these particulars in the case of our earth, and how 
its appearance changes in consequence as seen from the sun. 
' Equally, therefore, before we can properly define our sun 
as a member of our system, we must find out these parti- 
culars for it ; we must know how it also is floating in our 
hypothetical ocean, whether at rest or perpendicularly, and 
if neither, then the length of its day, to speak in earthly 
language, and the position of its pole-star must be deter- 
mined. But how is this to be accomplished ? We need 
scarcely say that it is to the spots on the sun that we must 
look for the solution of these problems. Fortunately, the 
sun, when examined with a telescope, is not the equally 
illuminated disc it appears to be to the unaided vision, and 
the spots visible on its surface may be likened to straws 
which show us the rate and direction of a stream, for no 
sooner were they discovered than it was observed that 
they had a motion across the sun. 



MR. CARRJNGTON'S KESEASCHES. 



Now let us return toour hypothetical ocean, and sec how cnxr. i 




Fir. .6— The Fanh.«n fromiht S,i.. (Summfr SolHi«. BOW. 11 London 

we can take advantage of this knowledge : if our half- 



SOLAR PHYSICS. 



. immersed sun were floatinf; quite uprightly, the spots 




Fic. it — 'I1ie tinh, xcn Ciom ihe Sun {Winter SoIiUli. nwn u London) 

carried round by its rotation would always keep at the 



UR. CAXaiKCTOirS RESEAKCHES. 



same height above the waves, from whatever part at the <TiAr. m. 
earth's annual joumeni^ the sua was observed. But this 
we do not find to be the case From two opposite points tjV^ ^f 
of the earth's orbit — the points it occupies in June and *f*Mu 
December — the spots are seen to describe stra^ht lines 
across the disc ; while, midway between these points, 
in September and March, these paths are observed to be 
sharply cur\'ed. in one case with the convex side upward, 
in the other with the convex side downward. A mooicnt's 
thought wilt show that these appearances can arise only 
from a tipping down of the sun's axis, the amount and 
direction of which can be ascertained by observing either 




the angle made by the parallel paths which the spots 
describe with the surface of our ocean, or the amount of 
cur\-ature of the curves, and by noting (he earths place 
when the lines are straighiest or the curves the most 
prononfies. It is thus found that the sun's axis inclines 
towards the point occupied by the earth in September. 

Here, then, we find Mr. Carrington sitting down to work 
in order to record every spot visible, whether small or 
great, on the day of observation. To get a view of the 

a he projected an image, as did Scheiner with that first 
of cquatorials, his Hdiotropium uUoicopimm, and as. 
doubtless, many of our readers have done, on a screen ; 




i 



38 SOLAR PHYSICS. 



CHAP. III. but how was the position of the spots to . be accurately 

recorded } There was in 1855 no Kew photoheliograph to 

do this ; but the question received at Mr. Carrington's hands 

a satisfactory solution almost as soon as suggested, and the 

eyepiece was armed with two cross wires, very nearly at 

right angles to each other, and inclined approximately 45° 

on each side of the parallel of declination. The why and the 

• wherefore of this beautiful contrivance are mathematically 

demonstrated and fully explained by Mr. Carrington. 

Carring' The telescope, when adjusted in declination so that the 

ton's ar- image of the intersection of the cross bars would fall nearly 

for finding o:i the Centre of the image of the sun, was clamped ; and 

position of ^^^ image of the latter was allowed to travel over the 

fixed images of the bars, the exact times of contact, true to 

the tenth of a second, at which the limbs and spots touched 

both bars being noted. Sometimes as many as thirteen 

spots were thus observed at three passages over one bar 

and two over the other. 

Nothing can surpass the wonderful patience with which 
Mr. Carrington — our English Schwabe — has thus collected 
thousands upon thousands of observations, or the con- 
summate skill with which they have been reduced. It is. 
perhaps, from the 166 plates in which Mr. Carrington 
with the most scrupulous care has represented the spots 
visible on each d«iy of observation, that his diligence will 
be most tangibly gathered. The exact position of each 
group on the sun in reference to its equator and the as- 
sumed prime meridian is given in one series ; in another 
all the observations of each group observed more than 
once are fully shown, the altered appearances of the 
spots being given as well as their different positions on 
the disc. 

And here wc approach one of Mr. Carrington's results, 
apparently a very simple one, but one that would amply 
repay him for all his labour were it to stand alone. 

All our text-books tell us that the sun turns on its 
axis, the period of its axial rotation having been deduced 



MR. CARRINGTON'S RESEARCHES. 



39 



from observations of its spots noticed indiscriminately chap. hi. 
in any part of the disc. But, from the time of Galileo, ~~, 
who made the period* of rotation about a lunar month, sun^s'rotJ- 
down to our own, authorities have differed very consi- ^^ ^ *^^ 
derably. Thus Grant, in his " History of Physical Astro- 
nomy," gives a period of 27d. 8h. (he quotes no authority). 
Laugier found 25'34d., and later observers have made it 
still less. 

Mr. Carrington now comes to the rescue, artd announces 
to the world a discovery of the utmost value. He tells 
us tfte spots travel at different rates ^ depending upon their 
distance from the equator^ either north or south, and 
that the different rates are bound together by a law, so 
that he is enabled to represent all the rates very nearly by 
the following formula : — 

86s - i6s'sin.i(/- i°) 
So that the sidereal rotation of the equatorial photo- 
sphere is accomplished in 30'86 days ; and of that at a 
latitude of 50° N. or S. — the highest point at which spots 
have been observed — in 2836 days. 



Proper 

motion of 

spots. 



We said of the photosphere : the sun itself, whether it be 
the glade-bedeckt world imagined by Sir W. Herschel, or 
the incandescent globe required by both the old and the 
new philosophies, has revealed none of its secrets to Mr. 
Carrington. But it is clear that it must be content with 
one only of these differing rates of motion : and the ques- 
tion is, what is it } Sir John Herschel, in an admirable 
article on sun-spots in the Quarterly Journal of Science, 
deals with this question. Mr. Carrington considers that 
the views of Professor (now Sir William) Thomson " On 
the Mechanical Energies of the Solar System " are sup- 
ported by his discovery, supposing that the sun itself 
travels more slowly than the equatorial photosphere. He 
remarks :— " In the absence of an impressed motion from 
some such external force, it would be expected that the 
currents of the surface of the sun would resemble those of 



40 SOLAR PHYSICS. 



CHAP. III. the earth's ocean and atmosphere, and be westerly and 
towards the poles in the tropical latitudes, and easterly in 
the higher latitudes ; the direction of rotation in such cases 
being the same, and the equatorial region in each the 
hottest." Besides determining anew the elements of the 
sun's equator — in other words, the position of the sun's 
pole-star — Mr. Carrington has put us in possession of an 
important fact regarding the minimum period of sun-spots. 

Contraction He detected " a great contraction of the limiting parallels 

^r^ds'^at between which spots were found previously to the mini- 
minimum mum ; . . . . and soon after this epoch the apparent 

spot period, commencement of two fresh belts of spots in high latitudes^ 
north and south, which have in the subsequent years shown 
a tendency to coalesce, and ultimately to contract, as 
before, to extinction." In Sir John Herschel's paper,^ to 
which we have before alluded, there is a passage which 
shows in a v^ry strong light the value of these remarks of 
Mr. Carrington's. In attempting to account for the phe- 
nomena of sun-spots by the presence of a nebulous ring, 
HerschcVs he writes : — " Let us suppose (and such a supposition has 

rin^Zor ^^^ been deemed inadmissible in attempting to account 
for the periodical return of meteors) the existence of an 
elliptic ring of vaporous, nebulous, or small planetary 
matter, which such a major semi-axis (4*979) as corre- 
sponds to a periodic time of each of its particles = irii 
years ; of such eccentricity as to bring its perihelion within 
the limits of the solar envelopes ; and revolving either in 
the plane of the ecliptic or in some other plane at a more 
considerable inclination of the sun's equator. Let it be 
further assumed (still in anlaogy with assumptions not 
regarded as unreasonable in the meteoriferous ring), that 
the distribution of the circulating matter in it is not 
uniform — that it has a maximum and minimum of density 
at nearly, but not quite, opposite points, and no great 
regularity of gradation between them. It is very con- 
ceivable that the matter of such a ring, introducing itself 
Quarterly Journal of Science, A^ril 1864. 



MR, CARRINGTON'S RESEARCHES, 41 



with planetary velocity into the upper and rarer regions chap. hi. 
of the sun's atmosphere at an incidence oblique to its 
regular and uniform equatorial drift, might create such 
disturbances as, either acting directly on the photosphere, 
or intermediately through a series of vortices or irregular 
movements propagated through the general atmosphere, 
should break its continuity and give rise to spots, con- 
forming in respect of their abundance and magnitude to 
the required law of periodic recurrence. If the change of 
density from the maximum to the minimum were gradual, 
but from the minimum to the maximum more abrupt, so 
as to allow the disturbances to subside gradually and re- 
commence abruptly — the fresh and violent impulse would 
be delivered first of all on a region remote from the 
equator (by reason of the obliquity of the ring), and would 
give rise to a recommencement of the spots in compara- 
tively high latitudes. 

" If the section of such a ring as we have supposed at Sectwfi of 
its aphelion were «i7, the period of irii years would be ^aphdion 
strictly carried out; the maxima and minima would sue- ni' 
ceed each other with perfect regularity, and the paucity and 
abundance of the spots in the several phases of the same 
period would follow a fixed ratio. But if not, the several 
parts of the ring would not revolve in precisely equal times 
— the period of irii years would be that of some 
dominant medial line, or common axis of all the sections 
in which a considerable majority of its matter was con- 
tained ; and the want of perfect coincidence of the other 
revolutions would more or less confuse without obliterating 
the law of periodicity, which, supposing the difference to 
be comprised within narrow limits, might still stand out 
very prominently. Now, it might happen that there were 
two such medial lines, or more copiously stocked ellipses, 
each having a maximum or minimum of density, and that 
their difference of periodic times should be such as to bring 
round a conjunction of their maxima in 56 or any other 
considerable number of years ; and thus would arise a 



42 SOLAR PHYSICS. 

ciiAP. III. phenomenon the exact parallel of Dr. Wolfs long period 

and his series of greater and less maxima." 

7 he value We have given this extract to show the enormous value 

^f ^y^^'y of a single well-ascertained fact ; and Mr. Carring^on may be 

fact, congratulated upon the possession of that sagacity which, 

by limiting his inquiry, has enabled him to produce such 

facts. But to show how wide is the field laid open to us by 

any facts connected with the sun, we may state that, in 

his last plate, Mr. Carrington gives two curves showing the 

variations of spot-frequency in the 11*2 years period dis- 

Curvcs of covered by Wolf, and the variations of the distance from 

^^ar^of ^^^ ^"" ^^ ^^ planet Jupiter, — and truly the curves run 
Jupiter's together in a very marvellous manner for a certain distance ; 
competed. ^^^ more recently still Dr. Balfour Stewart has attempted 
to account for the long sun-spot period of about fifty-six 
years, which in such a remarkable manner connects the 
increase of sun-spots and the frequency of aurorae. He 
remarks, that as two revolutions of Saturn are very nearly 
equal to five of Jupiter, we shall have every fifty-nine years 
the same planetary phenomenon repeated ; and he shows 
that the dates of the aphelion of the two planets come 
closest together about 1840, which is not far distant from 
1836, the date of maximum given by Professor Wolf. 
There will be more to say on these planetary connections 
presently. 

This work — this splendid addition to our astronomical 
literature and knowledge — is, we must state in conclusion, 
after all but a kind of hors (Tceuvre undertaken to fill up 
those parts of the day which were not required for the 
reduction of the nights' observations made for the Redhill 
Circumpolar Stars Catalogue. Looked at from any point 
of view, it reflects the highest honour, not only upon its 
author, but upon the Royal Society, who have aided its pub- 
lication by a grant, and upon the whole body of English 
amateur astronomers. It is well that all should know that 
there are such men as Mr. Carrington, who are content to 
give their time, money, energies, talents, everything they 



MR. CARRINGTON'S RESEARCHES. 43 



possess, to the pursuit of such studies, and, while Kew and chap. hi. 
Ely are daily registering the sun,^ so as to extend the use- 
fulness and gather fresh facts by means of the methods 
here laid down, we hope that some one will be encouraged 
by Mr. Carrington*s bright example to study the physical 
features of the spots apart from all theories, and present 
us with the detailed telescopic appearances which they 
present at intervals of — say — some half-hour or so. Who 
will volunteer } 

1 Alas ! this is no longer true ! to the lasting disgrace of British 
Science the daily photographic record has ceased. (1873.) 



M. FA YE'S FIRST THEOR V OF THE PHYSICAL 
CONSTITUTION OF THE SUN. 

CHAP. IV Little wonder is it that the physical constitution of that 
luminary in which modern science teaches us our very 
life itself centres, should be at present a subject of all- 
absorbing interest among philosophers ; and little wonder, 
too, is it, that in spite of all the aids now at the disposal of 
scientific men the riddle of the sun is yet unread. We 
have done so much, and gleaned so many facts at distances 
the very mention of which is almost meaningless to us, so 
stupendous are they, that we forget that our mighty sun, in 
spite of its brilliant shining and fostering heat, is still some 
ninety odd millions of miles removed ; that its diameter is a 
hundred times greater than that of our earth ; and that the 
yawning chasm we modestly call a sun-spot is yet large 
enough to swallow us up and half-a-dozen of our sister 
planets besides ; while, if we employ the finest telescope 
under the most favourable atmospheric conditions, we are 
only enabled to observe the various phenomena as we 
should do with the naked eye at a distance of 180,000 
miles. 

Surely our powerlessness to grasp the physics of the sun 
should surprise no one, if this fact alone be borne in mind, 
while the " willow-leaf controversy " shows us the difficulty 
our observers have, not in collecting facts, but in actually 
being sure of what they see. The very stupendousness of 



FAYE'S FIRST THEORY. 



45 



the scale on which the action is carried on on the sun's chap. iv. 
surface is another difficulty, although a fortunate one, for 
without it we should see nothing. Still our observers, 
our physicists, and our theorists are not dismayed ; and as 
a sign of the present activity, the question of solar physics 
has recently occupied the attention of our Royal Society 
and the French Academy of Sciences ; a searching re- 
duction of the Kew pictures and a notice of the physical 
aspect of the sun being presented to the former body by 
Messrs. De la Rue, Stewart, Loewy,^ and Professor Phillips, 
and two papers dealing with the physical constitution of 
the sun being presented to the latter by M. Faye.^ 

It is to the communications to the last-named body that 
we now wish to draw attention ; the more so as the part of 
it already published contains a very admirable historical 
notice of the progress of the inquiry. In a subsequent 
chapter we will discuss opinions at present held by our 
leading men of science. 

M. Faye's historical notice commences by a reference to 
Wilson's theory of a solid, dark, and relatively cold nucleus 
and brilliant envelope, which (as already stated) was 
afterwards adopted by Sir William Herschel, who further 
embellished it with a cloudy stratum, lying between the Herschd's 
nucleus and the photosphere. This stratum was eminently Stratum. 
reflective, though not self-luminous, like the photosphere. 
Wilson's gaseous eruption, the bursting of which through 
the photosphere caused a " sun-spot," was now held to break 
through the cloudy stratum as well, and this latter was held 
to give rise to the appearance of the penumbra. It will be 
seen that both Wilson and Herschel built up their solar 
theory on terrestrial models. The latter went so far as to 
imagine in our aurorae a resemblance to the photosphere, 
and among the functions which he attributed to the cloudy 
stratum was that of rendering our sun a habitable globe. 
Hence the perfect reflecting power which he ascribed to it. 

' The results of the Kew work will be stated in detail subsequently. 
' Comptcs Rcttdus^ vol. Ix. pp. 89 and 138, 1865. 



46 SOLAR PHYSICS. 



C HAP. IV . M. Faye, however, points out that, in addition to the the- 
ories now exploded, we owe to Wilson our first knowledge 
of two important facts : (i) That the spots are cavities ; 
(2) That the photosphere is neither solid nor liquid, but 
of a nebulous, gaseous structure. On the first point the 
contrary opinions held by La Hire, Lalande, and some 
physicists of our own time are strongly commented upon, 
astonishment is expressed at the hardihood of those who 
hold them, and the beautiful stereoscopic combination 
imagined by De la Rue is advanced in support of the 
unanimous testimony of all who have ever observed the 
sun through a telescope, or have brought the laws of per- 
spective to bear upon the inquiry. It is curious, however. 
1/ the spots that the crucial remark of Dawes, that if the spots were 
we should clouds we should have a notched limb whenever they 

^^^hM r^^^^^ ^ff ^^ dASQy is not alluded to. 
limb when On the sccond point, regarding the photosphere, it is 
they leave remarked that astronomers would sooner have asserted the 

or enter the 1 . r 1 1 • 

sun, small mean density of the sun and its enormous heat m 
support of the evidence of their telescopes, if they had not 
so long held to the theory of the cool and habitable globe 
underneath. So that Arago's deduction from his experi- 
ments on the polarization of the sun's light — a deduction 
which supported the theory of the gaseous nature of the 
photosphere from a new point of view — was doubly welcome. 
Many objections were raised against the validity of this 
new argument. Some objected to it as being merely a 
cabinet experiment (we know better than this now); 
others have met it with absolute and complete denial. 
Thus, Sir John Herschel has stated that, in consequence of 
Effect 0/ the enormous rugosity of the sun, the rays which reach us 

therugostty •11. 

of the sun. irom any portion near the limb are not necessarily rays 
which leave the sun at a small angle from the light-giving 
surface ; they may come from surfaces having all imagin- 
able inclinations to the ray. It is evident, therefore, that 
the light coming from the margin contains a mixture of 
rays polarized in every direction,— a condition of things 



FAYE'S FIRST THEORY. 47 

which would entirely vitiate Arago's conclusions. M. Faye chap. iv. 
thinks that if the illustrious " perpetual secretary " were 
still among us, he would reply, that at so great a distance 
a portion of the surface of any size, near the limb, would, 
in spite of these irregularities, affect a general direction, 
which would coincide with the mean surface to which is due 
the contour of the sun ; hence a general predominance of 
obliquity in a given direction for the generality of rays 
which enter the polariscope : consequently the rays should 
contain a certain proportion of light polarized perpendicu- 
larly to the plane of convergence, if the radiating body is 
solid or liquid. M. Faye states, in addition, that he has Fay/sexpf- 
lately been experimenting on globes of frosted silver, which '^/^J^" 
everybody will grant presents a rougher surface, relatively, silver 
than the sun. Still the polarization was very strongly ^ '^^' 
manifested towards the borders, and even on the parts 
much nearer the centre. 

A second objection is more important. Angstrom has 
shown that gases and vapours absorb the rays of a refran- Absorption 
gibility identical with that of the light which they them- ""f '^^^' ^y 
selves emit when brought to a state of incandescence ; 
and, taking advantage of this discovery, MM. Kirchhoff 
and Bunsen have shown that it is possible to reproduce, 
artificially, the principal lines of the solar spectrum by 
interposing the vapours of various metals between a sub- 
stance capable of giving out a continuous spectrum, and 
the eye. 

M. Kirchhoff has applied to the sun itself this excellent Alrchhofs 
laboratory experiment. We want a source of light giving ^*^ ' 
a continuous spectrum : this will be the photosphere. We 
want interposed metallic vapours : they will form the invi- 
sible atmosphere of the sun. The nature of these vapours 
will be determined by the solar rays absorbed, but incan- 
descent solids and liquids give a continuous spectrum, 
whilst gases and vapours furnish only one limited to a few 
bright bands: therefore the photosphere, far from being 
gaseous, as we have thought, and as Arago believed himself 



SOLAR PHYSICS. 



Kinhhoff', 

hypolkais 



to have shown experimentally, must be a solid, because 
it gives a continuous spectrum. So these two celebrated 
experiments are contradictory ; the polariscope says one 
thing, and the spectroscope says another, and many philo- 
sophers, forgetting the once vaunted experiment of Arago, 
have accepted the evidence furnished by the spectroscope. 

M. Faye affirms that this fundamental contradiction is 
one in appearance only, and he undertakes to prove this 
in the second part of his memoir. In the interim he affirms 
again that M. Kirchhoff's conception of a solid or liquid 
incandescent sun cannot by any possibility represent the 
actual facts. 




He remarks that if an actual solid or liquid condition be 
assigned to the photosphere, we must look elsewhere for 
the cause of the spots. This is what M. KirchhofT has 
done, reproducing Galileo's first impression, in spite of 
Galileo's recantation of it. Galileo reasoned as follows : If 
two neighbouring spots arc observed near the centre of the 
disc, with a bright interval between them, if the spots were 
protuberances this line of light would decrease as the spots 
approached the limb, and would soon disappear, because 
one of the -spuls, if it were a projection, would hide it. But 



FAVE'S FIRST THEORY. 



49 



CHAP. IV. 



Distribu- 
tion of 
spots. 



observation proves that this line of light remains constant 
till the spots disappear. For the last two centuries and a 
half astronomers have contemplated and measured without 
finding Galileo's remark wanting, and it is useless to remark 
that Wilson's argument, unknown to Galileo, has finally 
fixed our ideas as to the nature of the spots. 

M. Faye next refers to the distribution of the spots on 
the sun's surface. Since the time of Fabricius, Galileo, and 
Father Scheiner even, we have known that the spots are 
confined to two zones between the parallels of 30'' and 35° 
north and south, excepting an equatorial zone a few degrees 
wide, in which they very rarely show themselves ; hence the 
spots are in relation with the sun's rotation, and we have 
analogous phenomena in the case of the larger planets, our 
own earth included. 

Now if, as is the case with the planets, the polar regions 
of the sun are colder than the equatorial regions, there would 
be produced in 4ts atmosphere — always accepting the theory 
of the dark nucleus — currents analogous to our trade winds, 
and consequently cyclones capable of tearing the photo- 
phere, and even the cloudy stratum. On this hypothesis 
we can understand the narrow limits within which the 
s[>ots appear, reasoning by analogy from the monsoon 
regions of the earth, which are those infested by cyclones ; 
and further sun-spots would be produced by descending Sfotspro- 
currents, and not by ascending eruptions, as held by Wilson desalidii^ 
and Sir William Herschel. Sir John Herschel's beautiful 
theory, that the sun is actually colder at the poles, by reason 
of the smaller thickness of the atmosphere in the polar 
rqjions allowing a greater radiation of heat, is therefore 
next alluded to, and it is remarked that this brilliant 
conjecture has at once taken the sun-spot theory out of 
the domain of perspective, and rendered it capable of 
dynamical treatment. Still, M. Faye does not accept 
Sir John Herschel's suggestion : he holds, in the first 
place, that the rotation of the sun is too slow to produce 
the atmospheric flattening required by it ; and secondly, 

E 



currents. 



5^ SOLAR PHYSICS, 



rHAi'Mv. that the spots would affect, like our own clouds, a common 
movement of translation from the poles to the equator, — 
a movement not confirmed by modern observation. After 
alluding to the work of Mayer, Thomson, and others, — 
work which has enlarged the circle of our ideas, and 
banished from science the notion of a solar nucleus, 
dark and cool, — M. Faye promises in the next part of 
his paper to give the principal results of our modern 
work, and co-ordinate them, starting with the idea of a 
gradual cooling of an enormous mass, animated by a move- 
ment of rotation, and of an excessive temperature which 
maintains all the elements in a chaos of complete disunion, 
except at the liviit which separates the mass from the 
vacuum and cold of space. 



M. FA YE'S FIRST THEOR Y OF THE PHYSICAL 
CONSTITUTION OF THE SUN (continued). 



We said in the preceding chapter that the riddle of the 
sun was yet unread. A careful perusal of the second part 
of M. Payees very remarkable memoir would almost make 
us pause before we repeated the remark, not because we 
have accepted his solution — it is not to be disposed of so 
easily — but because it is one of the most important contri- 
butions to this subject that we have had for some time, and 
it is quite possible that the full discussion which it certainly 
will receive at the hands of our physicists may result in an 
extension of our present knowledge, and a firmer stand- 
point for further investigations. The second part of M. 
'Faye's memoir commences with a reference to the work 
done on the sun since Arago^s time, that work being 
classed as follows : — (i) Study of spots, facula^, and general 
surface of the photosphere. (2) The movement of rotation. 
(3) Phenomena exterior to the photosphere, observed during 
total eclipses of the sun. The latter are not referred to at 
any great length, as the photosphere is now in question 
principally, but it is shown that the presence of rose- 
coloured prominences on those parts of the sun where spots 
are never seen is a sufficient proof of the want of connec- 
tion between the phenomena. Schwabe's sun-spot period — 
the lengthening of the spots in the direction of the parallel 

— the slow gyratory motion observed by Dawes — the obser- 

E 2 



CHAP. V. 



Gyratory 

mmcmmt 

of spots. 



52 SOLAR PHYSICS, 



CHAP. V. vations of M. Chacornac — and the proofs brought foru'ard 
by Dawes, Secchi. and De la Rue of the height of the 
faculae, are mentioned, and reference is made to the work 
at Kew ; and here, before we proceed, we will give, as briefly 
as possible, the latest results obtained by the Kew observers, 
which were communicated to the Royal Society at its last 
meeting, — results which bear closely upon this part of 
M. Faye*s memoir. The materials at their disposal con- 
sisted of Mr. Carrington*s original drawings from November 
1853 to March 1861, and the Kew photographs taken con- 
tinuously since 1858. To Nature as exhibited in these 
drawings they have put the questions : — 

(i) Is the umbra of a spot nearer the sun's centre than 
the penumbra ; or, in other words, is it at a lower level } 

(2) Is the photosphere of our luminary to be viewed as 
composed of heavy solid or liquid matter, or is it of the 
nature either of a gas or cloud } 

(3) Is a spot (including both umbra and penumbra) a 
phenomenon which takes place beneath the level of the 
sun's photosphere or above it } 

Now if the umbra is appreciably at a lower level than 
the penumbra, we arc entitled to look for an apparent 
encroachment of the umbra upon the penumbra on that 
side which is nearest the visual centre of the disk. This 
is the phenomenon observed by Wilson. 
Encroach- Taking all the cases where an encroaching behaviour of 
^^^umbra!^ the umbra in a right and left direction has been perceptible, 
^6 per cent, are in favour of the hypothesis that the umbra 
is nearer the centre than the penumbra, while 14 per cent, 
are against it. Taking all available spots and distributing 
them into zones according to their distance from the centre, 
this encroaching behaviour is greatest when spots are near 
the border, and least when they are near the centre. 
Dealing with spots in high latitude only, and taking all 
those cases where an encroaching behaviour of the umbra 
in an up-and-down direction has been perceptible, 80*9 per 
cent, are in favour of the hypothesis that the umbra is 



FAYE'S FIRST THEORY. 53 



nearer the centre than the penumbra, while 191 per cent. chap. v. 
are against it " 

Here, then, we find the most recent discussion of these 
observations bearing out entirely the view held all along 
by astronomers, and adopted by M. Faye in the first part 
of his memoir. Mr. Carrington's recent book is the prin- 
cipal text chosen by M. Faye on which to base his remarks 
on the rotation. He gives a table showing the period of 
rotation determined from observations of spots in both 
north and south heliocentric latitudes up to 50", — a period 
varying from 25*06 days to 2846. The displacement of Transport 
the spots in latitude is especially dwelt upon, as they show p^e^^^ 
that the great currents which on our globe transport masses ^**atorJoes 
of air from the poles to the equator do not exist on the on the sun, 
sun, — a fact before referred to, and which tells equally 
against HerscheFs and Kirchhoffs hypothesis. In fact 
clouds or cyclones cannot make head against the rotation 
with a rapidity of 2,000 leagues a day (in lat. 35°), except 
they approach the equator with a rapidity comparable to 
that of their movement in longitude. Now, if Mr. Car- 
rington*s observations tend to enlighten us at all on this 
head, past the 15th degree of latitude, they testify to a 
movement from the equator to the poles in exactly the 
opposite direction to that required by these hypotheses. 

M. Faye then proceeds to give his explanation of the 
physics of the sun, taking as a start-point the fact that 
nothing really distinguishes our sun from the multitude of 
stars which shine in the heavens. Astronomers admit will- 
ingly that he is a star of mean size, of nearly white light, 
and slightly variable. We have, therefore, to deal with a 
phenomenon of considerable importance to us doubtless, 
but one after all of common, and indeed ordinary character, 
met with in the stellar universe. It is convenient, there- 
fore, that we should set out with the most simple and 
general idea, and the one most applicable to the stars 
taken as a whole. This idea is that of the successive 
reunion in vast agglomerations, under the influence of 



54 SOLAR PHYSICS. 



CHAP. V. attraction, of the matter of the materials first scattered 
through space. Hence two immediate consequences — 
(i) The destruction of an enormous quantity of vis viva, 
replaced by an enormous development of heat ; (2) a 
movement of rotation more or less slow for the entire 
mass. The calculation of the amount of heat developed 
in the formation of the sun has been made by M. Helm- 
holtz, by the aid of several plausible suppositions as to 
the numerical elements of the question. This calculation 
shows that it is easy to account for a duration of several 
Chemical millions of years, although chemical action would only 

incomLunt sustain the present radiation of heat for half the actual 

to maintain historical period (3,000 years). 
tempera- This internal heat, when it is a question of masses so 
ture. considerable, much surpasses that at which chemical action 
commences, but the cooling down in such a mixture of 
gases and vapours determines successive phases which 
M. Faye examines. In consequence of this cooling, in 
which conductibility exercises but an insignificant power by 
means of interior movements, a stable equilibrium between 
the different strata, analogous to that in our own atmo- 
sphere, in which the displacements are due to an exterior 
cause, will soon be established. Now, whatever may be the 
temperature of this homogeneous gaseous mass, its radiating 
power, confined to its surface — since each stratum possesses 
a special absorptive power for the rays emitted by the 
inferior layers — will be very small. The conducting powers 
being also fuller, the equilibrium of the entire mass will 
undergo but slow modifications, and unless new conditions 
be introduced, we cannot understand how such a mass can 
radiate the enormous quantity of heat, which seems to 
undergo no diminution in the course of ages. On this 
point M. Faye reasons as follows : " In fact, the tempera- 
ture of the surface of the sun is far from being so elevated 
as that interioroneof complete dissociation^ just mentioned. 
From M. Pouillet's measures of the actual intensity of 
1 A term borrowed from M. H. Sainte-Claire Devillc. 



FAYE'S FIRST THEORY, 55 



sun. 



solar radiation, Professor Thomson has found that the heat chap. v. 
emitted is but from fifteen to forty-five times greater than 
that obtained in the furnaces of our locomotives. So that umpera- 
the superficial temperature will not enormously surpass that ^^[,^1 ^^^ 
which we can produce in our laboratories, — a temperature, 
it is true, capable of producing the dissociation of a great 
number of substances, but which is still resisted by the 
more stable compounds.^ The comparison of the light of 
the sun with that of our most powerful artificial light- 
sources corroborates this deduction. 

" It results from this that, if the molecular and atomic 
forces of cohesion and affinity cease to act in the interior 
of the mass, they come into play on the surface, where, 
in a gaseous mixture of the most varied elements, the 
operations of these forces will give rise to precipitations 
(Herschel), clouds (Wilson), non-gaseous particles capable 
of incandescence, of which our brilliant terrestrial flames 
offer so many examples.^ Soon these particles, obeying 
the forces of gravity, will in falling regain the temperature 
of dissociation, and will be replaced in the superficial layer 
by ascending gaseous masses, which will act in the same 
manner. The general equilibrium, therefore, will be dis- 
turbed in the vertical direction only by an unceasing ex- 
change going on between the interior and the exterior — an 
exchange which was impossible in the preceding phase ; 
and as the interior mass thus placed in connection with the 
exterior is so enormous, we can conceive that the super- 
ficial radiation, fed incessantly by the vast reservoir of 
central heat, constitutes a phase of long duration and of 
great constancy. 

" Thus the formation of a photosphere — an apparent limit 
of the sun — is a simple consequence of cooling ; and as our 

* This question will be discussed in the sequel ; it is sufficient to 
state here that it is most probable that there is complete dissociation 
in the sun's atmosphere. (Note added 1873.) 

* Chemical action in a gaseous mixture may be set up in two ways ; 
by cooling if the mixture is at the temperature of dissociation, and by 
heat if the mixture is at an ordinary temperature. 



56 SOLAR PHYSICS. 



CHAP. V. assumption applies to all analogous bodies, the same phe- 
nomena must exist, or must have existed, in all the stars. 
" From this point of view the beautiful experiments of 
Arago's Arago and Kirchhoff are seen to be no longer contradic- 

Kirchhoff's ^^"^Y* ^^^ term incandescent gas was not used by Arago 

experiments in the sense attributed to it now. The flame he used was 

reconciled, ^j^^^ ^j- ^^ ordinary gas jet, and not the obscure one of 

a Bunsen's burner, or of a simple gas. Even the numerous 
savants who now admit, on the authority of a name justly 
illustrious, that the sun and the stars have liquid photo- 
spheres, have not perhaps considered that incandescent 
molecules diffused in a gaseous medium, itself heated to 
a high temperature, give a continuous spectrum, with the 
exception of the dark lines due to the absorption of 
the medium. 

"On the one hand, Arago's experiment conducts 
us to a correct conclusion, for light emitted from incan- 
descent particles floating singly in a gaseous medium 
can only be natural light, from whatever depth it comes, 
because it undergoes, at no incidence, sensible refraction 
by the surrounding medium. On the other hand, this 
medium exercises its absorptive powers, and determines 
in the continuous spectrum of the incandescent clouds the 
system of lines which belongs to its complex nature. 
Looking at it in this way, we can understand why the 

spectrum spectrum of the limb is identical with that of the centre (a 

^'^cmtrfo/ ^^^^ ^^^^ advanced by Forbes, and recently confirmed by 

ttiic Janssen after a much more detailed study ^), a result which 

iJcNtici. ^Quid certainly not occur if all the black lines of the 
solar spectrum arose exclusively from the interposition 
of the gaseous strata of the general medium, which may 
extend beyond the photosphere to a height still unknown. 
** The formation of the photosphere will now enable us 
to account for the spots and their movements. We have 
seen that the successive layers are constantly traversed 
by vertical currents, both ascending and descending. In 

* This point will be returned to. 



FA YE'S FIRST THEOR Y. $7 



this perpetual agitation we can readily imagine that chap. v. 
wJiere the ascending airrent becomes more intense^ the lumi- 
nous matter of the photosphere is momentarily dissipated. 
Through this kind of unveiling it is not the solid cold 
and black nucleus of the sun that we shall perceive, but 
the internal ambient gaseous mass, of which the radiating 
power, at the temperature of the most vivid incandescence, 
is so feeble in comparison to that of the luminous cloud 
of the non-gaseous particles, that the differences of these 
powers suffices to explain the contrast, so striking, of the 
two tones observed through our dark glasses."^ 

M. Faye, after referring to a similar opinion held by Nucleus of 
Father Secchi, whose thermo-electric measures of the spots cJoUrthan 
have shown that the nucleus of the spot radiates less thephoto- 
heat than the photosphere, goes on : — 

"But the most important phenomenon is assuredly 
that which has been so fully put in evidence by MM. 
Laugier and Carrington. Let us follow the same course 
of reasoning. From the continual exchange going on 
between the lower beds of the surface by means of 
vertical currents, we must conclude that the ordinary laws 
of rotation in a fluid mass in a state of equilibrium 
are strangely altered, since this equilibrium is con- 
stantly disturbed in a vertical direction. The ascending 
masses, which spring from a great depth, arrive at the 
top with a linear velocity of rotation less than that of 
the surface, because the layers whence they are derived 
have a smaller radius. Hence a general lagging in the 

' Is this in accordance with the received theory of radiation ? (Note 
added 1865.) 

This note was added to the article as it originally appeared, as the 
result of a conversation with my friend Dr. Balfour Stewart. I am 
the more anxious to state this, as to him belongs the credit of the 
objection, although, as it was some time afterwards put forward by 
Kirchhoff, the latter is now credited with it, although it was noticed 
by Faye, Comptes Reridus, vol. Ixiii. p. 235, 1866. The idea is this : — 
If the interior solar gases are feeble radiators, then, on the theory 
of exchanges, they must be feeble absorbers ; hence they will be in- 
competent to absorb the light coming through the h) pothetically 
gaseous sun from the photosphere on the other side. (1873.) 



58 SOLAR PHYSICS. 



CHAP. V. movement of the photosphere, although this lag is com- 
pensated for the whole mass by the descending currents 
in such a manner that the fundamental law of areas is 
satisfied. In the same way our own atmosphere does not 
exactly follow the law of rotation of a mass in equilibrium, 
but the effects are all different because it rests upon a 
solid (or liquid) surface. If the photosphere lags behind 
the general rotation, the lower beds should, by way of 
compensation, be in advance of this movement. From 
Motion of this opposition it results that, although the photosphere 
cH^n/s^in ^^^^^ havG a feeble tendency to approach the axis of rota- 
thesuH. tion by flowing superficially towards the poles, the contrary 
tendency will be apparent in the lower strata, which will 
approach the equator. The phenomena will take place 
as if the start-points of the vertical currents belonged to 
an interior surface farther from the poles than from the 
equator ; and if this ideal surface of emission be sphe- 
roidal, for instance, its depth, and consequently the retarda- 
tion of the photosphere in successive zones, would vary 
nearly as the sine squared of the latitude. Now this is 
exactly what Mr. Carrington's empirical formula would 
give if the breach of continuity, to which M. Babinet has 
justly objected, were corrected by replacing the power \ 
of the sine, by f or 2} I find, in fact, that the observa- 
tions are as well represented by the formula : — 

Daily motion = 862' — 186' sin ^L 
" Here, however, facts cease to guide us ; the law of these 
variations is not really known. The variety of spots in 
the first S"* of the equatorial zone, and in the polar zone, 
which commences at 35", does not permit us to determine 
at present the algebraical form of this variation. Here, 
then, is the problem which Mr. Carrington has bequeathed 
to us, and which we must at once attack with all the re- 
sources of science. It is to this part of the theory that 
will henceforth attach the distribution of spots, the phe- 
nomena of their periodicity, and the slight difference of 

* Comptfs Rendus^ Sept. 12, 1864, p. 481. 



FA YE 'S FIRST THEOR V. 59 



temperature which seems to exist between the poles and chap. v. 
the equator. .... 

** As to the faculae — luminous ridges which invariably 
indicate the near formation of a spot — iAey are evidently 
diUy like the spots, to ascending currents. The photo- 
sphere is not a level surface in a mathematical sense ; it is 
the limit to which the ascending currents transport, in the 
general fluid mass, the physical or chemical phenomena of 
incandescence. But although the phenomenon, taken as 
a whole, affects a remarkable regularity, since the brilliant 
surface appears to us perfectly spherical, we can imagine 
that a more than ordinary rapid local afflux may exceed 
this limit, and carry the luminous clouds a little above the Facula 
general level. Hence the inequalities cited by Sir J. ^fg^^^^^" 
Herschel in his objections to Arago's experiment — inequali- rounding 
ties confined, like the spots, to certain regions. From the ^sp^^g 
fact alone that faculae are elevated to a greater height in the 
general medium, their movement ought to be somewhat 
slower than the corresponding zone of photosphere ; hence 
a tendency to appear to follow the spots — that is, to be 
to the left of them — than to incline over the spots when 
the impulse of the local current has ceased, and leaves 
the spots themselves to disappear in the rapid inrush 
of photospheric clouds." 

We will here, before we print the summary of M. Faye's 
memoir, give another extract from a paper recently read 
at the Royal Society. It will show that the most rigid 
reduction by De la Rue, Stewart, and Loewy, of the 
best observations that we possess, most of them taken by 
the sun himself, tends exactly to the results just stated 
by M. Faye : — 

"The authors next endeavoured to answer the follow- 
ing questions : — Is the photosphere of our luminary to be 
viewed as composed of heavy solid or liquid matter, or is 
it of the nature either of a gas or cloud } It was observed 
that the great relative brightness of faculae near the limb 
leads to the belief that these masses exist at a high ele- 



6o SOLAR PHYSICS. 



CHAP. V. vation in the solar atmosphere, thereby escaping a great 
part of the absorptive influence which is particularly strong 
round the border; and this conclusion was confirmed by 
certain stereoscopic pictures produced by Mr. De la Rue, 
in which the faculae appeared greatly elevated. It was 
remarked that faculae often retained the same appearance 
for several days, as if their matter were capable of re- 
maining suspended for a time; .... but of 1,137 cases, 
584 have their faculae entirely or mostly on the left side, 
508 have it nearly equal on both sides, while only 45 have 
it mostly to the right. It would thus appear as if the 
luminous matter, being thrown up into a region of greater 
absolute velocity of rotation, fell behind to the left ; and 

Facuhus we have thus reason to suppose that the faculous matter 
matt^ which accompanies a spot is abstracted from that very 

taken from r i » r i i 

the place portion of the suns surface which contains the spot, and 
^^^th^^to^^ which has in this manner been robbed of its luminosity. 

** Again, there arc a good many cases in which a spot 
breaks up in the following manner: — A bridge of luminous 
matter of the same apparent luminosity as the surround- 
ing photosphere, appears to cross the umbra of a spot 
unaccompanied by any penumbra. There is good reason 
to think that this bridge is above the spot ; for were 
the umbra an opaque cloud, and the penumbra a semi- 
opaque cloud, both being above the sun's photosphere, it is 
unlikely that the spot would break up in such a manner 
that the observer should not perceive some penumbra 
accompanying the luminous bridge. Finally, detached 
portions of luminous matter sometimes appear to move 
across a spot without producing any permanent alteration. 

*' From all this it was inferred that the luminous photo- 
sphere is not to be viewed as composed of heavy solid or 
liquid matter, but is rather of the nature either of a gas 
or cloud ; and also that a spot is a phenomenon existing 
below the level of the sun's photosphere. 

"The paper concluded with theoretical considerations 
more or less probable. Since the central or bottom part 



FAYE'S FIRST THEORY. 



6i 



of a spot is much less luminous than the sun*s photo- 
sphere, it may perhaps be concluded that the spot is of a 
lower temperature than the photosphere 

" Finally, the authors propose the following question : — 
May not the falling behind of faculae be the physical re- 
action of the proper motion of spots observed by Carring- 
ton, so that, while the current passing upwards falls behind, 
carrying the luminous matter with it, the current coming 
down moves forward, carrying the spot with it ; and may 
not this atrrefit coming from a colder region account for the 
deficient luminosity which characterizes a spot ? " ^ 

Here we have the fundamental point of difference between 
M. Faye and the Kew Observers. We will now give the 
conclusion of M. Faye's memoir, which gives a rhum^ of 
the three middle phases of sun-life. 

(i) T\i^ phase of complete dissociation (that of the plane- 
tary nebulae T), in which the heat decreases from the centre 
to the circumference. This state is susceptible of a par- 
ticular equilibrium ; the radiating power is very feeble ; 
the light is purely superficial, since that of deep strata is 
capable of absorption by those at the surface. The spec- 
trum is probably reduced to a number of brilliant bands, 
separated by extensive dark intervals.^ 

(2) Cooling of the external strata to such an extent that 
the action of certain molecular affinities becomes possible ; 
formation of a photosphere, a kind of superficial laboratory, 
which determines the apparent outline of the mass ; con- 
siderable radiating power of light and heat. The emitted 
light comes from a considerable depth in the photosphere ; 
the spectrum of the preceding phase is inverted ; the light 
is not sensibly polarized at any angle of emergence. The 
enormous flux of heat from the photosphere is kept up at 
the expense of the whole mass, by the action of ascending 
and descending currents, which are established between the 

* It must not be forgotten that in his paper Faye considers a spot as 
the seat of an uprush. (1873.) 

* This looks like divination, if we suppose M. Faye to be ignorant 
of Mr. Huggins' discovery. (Note added 1S65.) 



CHAP. V. 



The thrcf 

middle 

phases of 

sun-life. 



62 SOLAR PHYSICS, 

CHAP. V. lower strata and the periphery — currents impossible in the 
preceding phase. This second phase should occupy a con- 
siderable period of time, and present a great fixity in its 
phenomena. If the photosphere is locally wanting, the 
light and heat are reduced in that part in the ratio of the 
radiating power of the photosphere and the general gaseous 
medium. The movement of rotation is not executed 
exactly " bodily," as in the preceding phase, in which the 
conditions of equilibrium were nearly those of a fluid mass : 
the surface moves more slowly than the entire mass, in 
consequence of the antagonism of the forces which disturb 
the equilibrium. The superficial phenomena reveal the 
character of the intcrmittence. 
Phase (3) When, in consequence of cooling, the vertical currents 

precedmq begrin to decline, when the entire mass successively contracts 

extinction. . . 

to a sufficient mean density, the photosphere becomes very 
thick, and takes at the surface a liquid or pasty, and finally 
a solid consistency. Then the communication with the 
central mass is intercepted ; the cooling of this mass is 
effected only in consequence of the conductibility of the 
liquids more or less pasty: that of the liquid or solid crust 
makes rapid progress at the superficies ; the rotation which 
has been accelerated is regulated, the phenomena of spots 
and faculse have disappeared, and the figure is that of a 
fluid mass in equilibrium under the action of interior forces. 
The intensity of the radiation decreases rapidly: that 
emitted obliquely is strongly polarized ; the preceding spec- 
trum does not change sensibly in appearance, but it also 
presents dark lines due to the real atmosphere, which is 
henceforward distinct from the body of the sun itself. The 
spectrum of the limb differs notably from that of the centre 
in the number and darkness of its lines. 

After this phase comes extinction : this is the geological 
phase.^ 

' This analysis of M. Faye's memoir is referred to by him in a 
subsequent communication to the Paris Academy : Comptes Rendus^ 
vol. Ix. p. 468. 



THE SUN AS A TYPE OF THE MATERIAL 

UNIVERSE} 



It is not necessary for our present purpose either that we chap. vi. 
should re-open the discussion as to the real discoverer of 
the solar spots, or that we should attempt to realize the 
-strange and overwhelming mixture of wonder and awe, not 
to say delight and terror, with which the announcement 
must have been received. Man with a wondrous " optick- 
tube " had at last dared to peer into the secrets of the sun, 
and had, all unconsciously, by so doing dealt a deathblow 
at the fundamental Aristotelian doctrine of the immuta- 
bility and incorruptibility of the heavens. The secret had 
been surprised ; the sun was no longer the exemplar of 
spotless purity. 

It is not astonishing, therefore, that whether we regard 
Galileo or Fabricius or Scheiner as the real discoverer, the 
secret was kept for many months before either of them 
gave it to the world ; or that the latter, a Jesuit, was only 
permitted by his ecclesiastical superior — who, so runs the 
record, remarked to him that he had read Aristotle's 
writings from end to end many times without finding any 
mention of solar spots — to publish his discovery at last 
under a nom de plume. These facts tell as strongly as 
anything can do of the mixed emotions (^f those pioneers 

* A joint communication to MacmillatCs Mas^aztne by the author 
and Dr. Balfour Stewart, F.R.S., who has permitted me to include it 
here. —J. N. L. 



64 



SOLAR PHYSICS, 



CHAP. VI 



in the field of solar research. The secret divulged, how- 
ever, the Schoolman was soon merged in the. investigator, 
and the problem was attacked with a closeness and ardour 
which are almost models for modern observers. Witness 
Galileo's first letter to Welser, the chief magistrate of 
Augsburg, dated May 4, 161 2, and Scheiner*s last to the 
same personage, dated July 25, 161 2, under the signature 
of Apelles latens post tabulam. 

It would seem, indeed, that everything which could be 
reaped by the instruments at their command was imme- 
diately garnered. The motion of the spots across the sun's 
disc from east to west ; the period in which they performed 
a complete circuit ; their changes from day to day ; the 
fact that they appeared for the most part in two zones, one 
north, the other south of the equator, — are samples of the 
secrets which the sun was at once compelled to yield up. 
What the spots were not was a question amply and closely 
discussed both by Galileo and Scheiner ; but as to what 
they were, agreement was more difficult: Galileo at one 
time declared for clouds in an invisible atmosphere of the 
sun, Scheiner for a density and opacity equal to that of the 
moon — in fact for planets separated from the sun*s surface, 
are planets, and revolving round him like Mercury and Venus. 

From the time of Galileo to 1769, or during nearly a 
century and a half, our knowledge was not increased by 
any new fact of importance, although in 1630 Scheiner 
managed to write a book of 784 pages ^ on the work which 
had been done in the two decades which had then elapsed 
since the discovery. It is true that Dclambrc has declared 
that there is not matter in this ponderous folio for fifty 
pages, but we hold that Delambre's dictum is harsh to a 
degree, and that when he made it he had entirely left out 
of sight the conditions under which the book had been 
written. 

In the year 1769 there was a very large spot visible upon 

^ ** Rosa Ursina, sive Sol ex admirando facularum et maculamm 
suanmi," &c. 



Galileo 

thinks the 

spots are 

clouds^ 
Scheiner 
that th,y 



THE SUN A TYPE OF THE MATERIAL UNIVERSE. 65 



the sun, and Dr. Wilson, of Glasgow, observed it very chap. vi. 
carefully, and demonstrated subsequently ^ that the spot 
was a cavity — a conclusion which, although combated by 
Lalande at once, and by others in quite recent times, main- 
tained and still maintains its ground. He also showed that 
the surface of the sun was probably of a cloudy nature. 

Wilson, the author of the important observation to 
which we have just referred, was also the first to put 
forward an elaborate theory of the origin and nature of 
sun-spots which much influenced the subsequent work till 
quite recently. These theories, subsequently taken up by 
Bode and Sir William Herschel, possess, however, but an 
historical interest, and it is no part of our present purpose 
to enlarge upon them. It must suffice to say that they 
were based on the assumption that the sun itself was a 
habitable, cool, glade-bedeckt globe beneath the luminous 
atmosphere, and that the appearance of a sun-spot was 
due to a gaseous eruption breaking through the cloudy 
envelopes of the solid globe : while La Hire held that they 
were purely surface-phenomena, and Lalande, that they 
were actual elevations. 

« « « « « 

From the description of the telescopic appearance of the 
sun given in Chapter H., it is obvious that the surface of 
the sun is uneven^ and that change of form is perpetually 
going on : these are conditions impossible in either a liquid 
or solid surface, such as land or ocean, but possible in a 
surface of cloud or gas. 

The cloud-like nature of the sun's surface follows, more- 
over, from the nature of the sun's light. This increase 
of our knowledge we owe to those immortal discoverers 
Kirchhoff and Bunsen, whose wonderful generalization of 
the results of spectrum analysis has given the present cen- 
tury a new fulcrum on which to move the great unknown 
by the lever of inquiry, and bring it into the light. 

* " Phil. Trans.," 1774. See ante^ page 10. 

F 



66 SOLAR PHYSICS, 



CHAP. vr. Their beautiful discovery not only enables us to define 
the sun as the nearest star, and to detect some ten terres- 
trial elements as existing in a state of vapour in its sur- 
rounding, absorbing, and therefore cooler^ atmosphere; but 
it enables us to state, as a proved fact, that the light of the 
sun proceeds from solid or liquid^ particles in a state of 
intense incandescence or glowing heat. 

We shall shortly have occasion to refer again to this 
method of research : the more recent work regarding the 
Motion of spots demands attention, however, beforehand in order 
^^^'' that we may follow as much as possible the order of time. 
It has already been stated that the early observers detected 
that the apparent motion of the spots was due to the real 
motion of rotation of the sun. But this account we now 
know is not all the truth. In addition to this apparent 
motion they have a real motion of their own of such a 
nature that the nearer a spot is to the sun's equator the 
faster it travels ; in fact the rate of this proper motion de- 
pends upon the latitude of the spot. This was one of the 
chief results deduced by Mr. Carrington from an elaborate 
daily investigation of the sun extending over six years, — a 
stupendous work, unsurpassed in the acumen and patience 
brought to the task, and rarely equalled in the results 
achieved. 

This discovery of the proper motion of the spots at once 
explained the strange discrepancies in the time of the sun's 
rotation as given by different observers, — discrepancies so 
great that Delambre declared it was useless to continue 
observations. 

Mr. Carrington's work did not stand alone about this 
time. The great Schwabe had previously determined that 
Sun-spot if the spotted area were taken at any one time, its amount 



teiiod. 



varied from year to year, — that is, that the spots themselves 
were periodical ; having periods of maximum and periods 
of minimum, the interval between two maximum or mini- 

* Or densely gaseous, according to Frankland's researches on 
liydrogen, and later work. (Note added 1873.) 



THE SUN A TYPE OF THE MATERIAL UNIVERSE, 67 



mum periods being about eleven years. The lamented chap. vi. 
Dawes and Father Secchi largely increased our knowledge 
of the solar surface, the latter determining specially that 
there was less heat radiated from a spot than from the 
general surface. 

Some time after Mr. Carrington's book appeared, M. 
Faye took up the question of solar physics with his usual 
elaborate treatment, and communicated to the Paris 
Academy of Sciences two papers of great value, in which, 
inter alia, he broached a new theory to account for the 
observed phenomena, and especially to explain the dark 
appearances presented by the spots. 

M. Faye regards the interior of the sun as consisting of No com- 
the original nebula, from which our whole system has been ^^"^^^^^^ 

** / •' can trxi.if 

slowly condensed, in a state of dissociation ; that is, at such in the 
an intense heat that chemical combinations are impossible ; '^7«I//^ 
and he looks upon the photosphere as the surface at which 
this heat is so acted upon by the cold of space as to allow 
chemical combinations and solid and liquid particles to 
exist. He goes on to remark that, if the molecular and 
atomic forces of cohesion and affinity cease to act in the 
interior of the mass, they come into play on the surface,^ 
where, in a gaseous mixture of the most varied elements, 
the operations of these forces will give rise to precipitations 
(Herschel), clouds (Wilson), and non-gaseous particles 
capable of incandescence, of which our brilliant terrestrial 
flames offer so many examples. These particles, obeying 
the force of gravity, will, in falling, regain the tempera- 
ture of dissociation, and will be replaced in the superficial 
layer by ascending gaseous masses, which will act in the 
same manner. The general equilibrium, therefore, will 
be disturbed in the vertical direction only by an un- 
ceasing exchange going on between the interior and the 
exterior. 

Having in this manner accounted for the photosphere 

» I shall show in the sequel, from my own researches, that this is 
most probably not the case. (1873.) 

K 2 



68 SOLAR PHYSICS. 



CHAP. VI. and for the incessant change which is observed, M. Faye 
goes on as we translate him : — 

" The formation of the photosphere will now enable us 
to account for the spots and their movements. We have 
seen that the successive layers are constantly traversed 
by vertical currents, both ascending and descending. In 
this perpetual agitation we can readily imagine that 
ivhere the ascending current becomes more intense the 
luminous matter of the photospftere is momefttarily dissi- 
pated. Through this kind of unveiling it is not the 
solid cold and black nucleus of the sun that we shall 
perceive, but the internal ambient, gaseous mass." 

Faye states ^ . , . . , , 

that the In this quotation we have the two most important points 
^^^sh^ • ^^ ^' ^*^y^*^ theory ; namely, that the spots are caused by 
an uprush, and that their dark appearance is due to feeble 
radiation from a gaseous surface. 
Thefacula M. Faye also considers that the faculae, like the spots, 
^^'^' are due to ascending currents, and he then attempts to 
account for the proper motion of the spots by the ascend- 
ing currents : — ** From the continual changes going on 
between the lower beds of the surface by means of ver- 
tical currents, we must conclude that the ordinary laws 
of rotation in a fluid mass in a state of equilibrium arc 
strangely altered, since this equilibrium is constantly 
disturbed in a vertical direction. The ascending masses 
which spring from a great depth arrive at the top with a 
linear velocity of rotation less than that of the surface, 
because the layers whence they are derived have a smaller 
radius. Hence a general lagging in the movement of the 
photosphere." 

These remarks of M. Faye will be fdund in the Comptes 
Rendus for i6th and 23d Jan. 1865.^ During the same 
month, a paper* was read at the Royal Society, in which 
certain results derived from the photographs taken at 

» See also Chapters IV. and V. 

« " Researches on Solar Physics." By Warren De la Rue, Balfour 
Stewart, and B. Loewy (Proc. Royal Society, vol. xv. p. 37). 



THE SUN A TYPE OF THE MATERIAL UNIVERSE. 



69 



Kew, and certain theories based therefrom, were discussed. 
We limit ourselves to the two most typical passages in this 
paper : — 

" Since the central or bottom part of a spot is much less 
luminous than the sun's photosphere, it may perhaps be 
concluded that the spot is of a lower temperature than the 
photosphere. . . . 

" May not the falling behind of faculse " (ample evidence 
of which is given in the paper) '* be the physical reaction 
of the proper motion of spots observed by Carrington ? 
so that while the current passing upwards falls behind, 
carrying the luminous matter with it, the current coming 
down moves forward, carrying the spot with it; and 
may not this current coming from a colder region account 
for the deficient luminosity which characterizes a spot } *' 

We see at once that on these points there is a perfectly 
clear issue between the two theories. M. Faye holds the 
spot to radiate feebly because it is Iiotter — in fact because 
it unfolds to us the interior of the sun in a state of dis- 
sociation. The Kew Observers hold that it is less lumi- 
nous because it is colder. Again, M. Faye holds that a 
spot is due to an uprush : the Kew Observers, that it is 
due to a downrush. 

At the outset there were many arguments against 
M. Faye's hypothesis. The law of exchanges was utterly 
against his idea of the darkness of a spot,^ for if it were 
the interior of the sun which we saw, and its radiation 
were feeble, then its absorption would have been equally 
feeble and the sun would be spotless ; for where the photo- 
sphere was torn away on the side nearest us, we should 
be able to see, through the sun, the lower surface of the 
photosphere on the opposite side. 

Again, the ai^uments in favour of an uprush, in the case 
both of spots and faculae, are not very clear, nor have 
we a satisfactory explanation of the falling behind of the 
faculae. But we had not long to wait for facts which, as 

* See note on p. 57, and Comptes Rtndus^ vol. Ixiii. p. 234, 1866. 



CHAP. VI. 



Difference 

between 

Af. Fayis 

theory^ and 

that of the 

Kew 
Observers. 



70 SOLAR PHYSICS. 

CHAP. VI. far as we can see, have entirely settled the question. First, 
as to the downrush into a spot. In 1865 two observers — 
one in France, the other in England — carefully observed 
the fine spots from time to time visible on the sun's disc in 
that year ; and the obser\'ations of both tend to show the 
absolute certainty that if spots are not caused by down- 
rushes, they are, at all events, fed by them. 

Let us hear the French observer first : ^ ** La rapidite des 
changcments est telle, que Ton peut suivre dans une mfime 
journee des courants des matieres photosphiriques se 
precipitant dans le gouffre principal en y transportant les 
petites taches voisines ; celles-ci en s'ajoutant k la grande, 
augmentent son ouverture et prouvent ainsi que la masse 
entiere de cette portion de T^corce solaire est transportee 
par ce courant" 
Visual The evidence derived from a spot observed in the next 

evidence as niQ^th througfh London foe: is not less conclusive.* The 

to d(nvn' 1 

rush, spot had a tongue of facula stretching half-way over it. 
When the observation commenced at 1 1.30 on April 2, this 
tongue of facula was extremely brilliant ; by I o'clock it 
had become less brilliant than any portion of the pen- 
umbra : at the same time the faculous mass seemed to be 
giving out at its end, veiling the umbra gradually with a 
kind of stratus cloud evolved out of it, which after a time 
again condensed into masses resembling the willow-leaves 
in the penumbra, only less distinct. 

The argument for the downrush is to be found in the 
fact of the diminution of brightness; accepting as proved, 
first, that the faculae are higher than the general surface, 
and, secondly, that a spot is a cavity. But it does not 
wholly depend upon this, for the masses or granulations on 
the general surface of the sun appear to lengthen out when 
they reach the penumbral region, as if they were acted 

* M. Chacornac, " Bulletin des Observations faites 2l Ville-Urbannc. 
Groupes des Taches Solaires," 6th March, 1865. 

» " Observations of a Sun-Spot," by the author (Monthly Notices 
of the Royal Astronomical Society, vol. xxv. p. 236). Sec anity pp. 
24- 31, and Comptcs RendMS,\o\, Ixi. p. 397. 



THE SUN A TYPE OF THE MATERIAL UNIVERSE, 71 



upon by a current, and this may also explain the constantly chap. vr. 
observed difference in the shape of the cloud masses on 
the general surface and in the penumbrae. In this con- 
nection it is worthy of remark, that when a solitary willow- 
leaf is seen over the centre of a spot,^ it is often observed 
to be nearly circular, as if its longer axis were tipped 
down. It is fair to add, however, that observations of the 
requisite delicacy can be very rarely made, owing to the 
many coincident conditions necessary. 

The fact that a spot is due to absorption has next to be 
considered. On M. Payees theory, as it will doubtless have 
already suggested itself to the reader, could a sun-spot 
be observed by means of a spectroscope, — as, by hypothesis^ spectre- 
we have radiation from a eras in a state of dissociation, — the "^'iT*'' 
resultmg spectrum would be a gaseous one ; that is, it 
would consist of bright lines. We, in fact, should get from 
a spot a spectrum absolutely different from that which 
belongs to the light emitted from the general surface, 
the latter being a band of rich colour going from red 
through yellow, green, blue, indigo, to the in tensest 
lavender, crossed by innumerable black lines of different 
intensity, the former consisting only of three or four thin 
bands of light, located in the green portion of the spectrum. 

On the absorption-hypothesis there would be none of 
these bright lines ; we should get a spectrum in the par- 
ticular region of the spot similar to the average solar one, 
but showing evidence of greater absorption. This was put 
to the test in 1 866.^ 

The method adopted was to apply a direct-vision spec- 
troscope to a 6i-inch equatorial, so that it was possible 
to observe at one time the spectra of the umbra of a spot 
and of the adjoining photosphere or penumbra. 

* See ante^ p. 28. 

* "Spectroscopic Obser\'ations of the Sun," by the author (Pro- 
ceedings of the Royal Society, vol. xv. p. 256). (This is given in extenso 
further on.) 



72 SOLAR PHYSICS. 



c HAP. VI. On turning the telescope and spectrum-apparatus, driven 
by clock-work, on to the sun, the solar spectrum was 
observed in the field of view of the spectroscope with its 
central portion (corresponding to the diameter of the umbra 
falling on the slit) greatly enfeebled in brilliancy. 
Selective ^^^ ^^^ absorption-bands visible in the spectrum of the 

absorption photosphcre, above and below, were visible in the spectrum 

inten^fied ^^ ^^ ^P^^ » ^^'^ ^^^^ appeared thicker witere they crossed the 
spot-spectrum} There was not the slightest indication of 
any bright bands. 

The dispersive power of the spectroscope employed was 
not sufficient to enable it to be determined whether the 
decreased brilliancy of the spot-spectrum was due in any 
measure to a greater number of bands of absorption. 

The Royal Society at once recognized the importance 
of this discovery, although it was put forward with much 
hesitation, as the instrument employed was not of sufficient 
dispersive power, and the spot itself was not a very favour- 
able one for the experiment. A larger instrument has 
now been constructed, and detailed observations are now 
about to be commenced under the auspices of that body. 
In the meantime, however, this settlement of the long- 
debated question has recently been entirely endorsed by 
Mr. Huggins, whose discovery of the physical constitution 
of nebulae, and spectroscopic observations of the fixed 
stars, make his opinion of the greatest possible weight. 

We have thus, as briefly as possible, traced up our 
knowledge of the sun's surface from the times of Galileo 
to our own. That surface, we have learnt, is of a cloudy 
nature, the light and heat being derived from the solid 
incandescent particles of which the clouds are composed. 
Further, there are exchanges perpetually going on between 
the cooler exterior and the interior. The descending 
current is accompanied by a spot, the ascending one by a 

^ A diagram of the appearance observed when more dispersion is 
used than I then employed is given further on. 



THE SUN A TYPE OF THE MATERIAL UNIVERSE, 



73 



facula ; and finally, the dark appearance of a spot, like the 
darkening of the limb, is due to the absorptive properties 
of the sun's atmosphere. 

Let us, for one moment, compare the sun's envelope 
with our own, and observe the action of the latter when 
the sun is withdrawn. 

The general surface of the ground is a good radiator. 
On the other hand, the atmosphere is at once a feeble 
absorbent and a feeble radiator. When the sun's influence 
is withdrawn from the earth's surface, and the sky is clear, 
the general surface of the ground and the leaves of plants 
give off their heat, which is radiated into space unimpeded 
by the very feeble absorbing power of the air ; on the 
other hand, the air, being a feeble radiator, gives back 
little or nothing in return. 

As far as radiation is concerned, therefore, the ground 
and leaves get rapidly cooler, nor is this loss of heat made 
up by any other process. Little or no heat can reach the 
cooled surface by conduction, for ground, leaves and air 
are bad conductors. Further, convection does not operate, 
for the particles of air next the cooled surface becoming 
cooler themselves become also heavier, and remain where 
they are. There is, therefore, no hindrance to the cooling of 
the earth's surface, which in its turn cools the air in contact 
with it until the air has reached so low a temperature that 
it cannot longer retain all its vapour. Part of the vapour 
is, therefore, deposited as moisture (or hoar frost if the 
temperature be below freezing-point), on the surface of the 
ground and the leaves of plants; and this is the expla- 
nation of dew and hoar frost, which we get when there 
is free exposure to the open sky. If there be cloud, a 
glass frame, matting, or any obstacle in the shape of a 
good radiator, interposed between the body and the sky, 
there will be no deposition of dew, because a quantity 
of heat will be derived from the radiator which has been 
interposed. Heat will therefore be lost very slowly, and 
moisture will not be deposited. It must be borne in mind 



CHAP. VI. 



Analogy 
between 
solar and 
terrestrial 
radiative 
pheno- 
mena. 



74 



SOLAR PHYSICS. 



CHAP. vr. that the presence of cloud makes an essential difTerence. 
We may suppose something equivalent to the deposition 
of dew, or at all events great radiation, to be taking place 
on t/ie npter surface of the cloudy not under the cloucL The 
heat of the bodies is retained in the latter region, the 
radiation being diminished, or rather compensated, by 
counter-radiation. It may be instructive to place ourselves 
in imagination above the surface of such a cloud, the sun 
being withdrawn, and consider for a moment what probably 
takes place. The small deposited particles, being great 
radiators, will rapidly get colder than the surroundii^ air ; 
they will, at the same time, cool the air around them ; 
and the air, being cooled, and thereby rendered heavier, will 
descend. There %vill thus be descending currents of air. 
But descending convection currents are naturally accom- 
panied with ascending ones. There will therefore be cucend- 
ing currents, conveying upwards some of the comparatively 
warm air from below. It is not impossible that such 
currents may assume in nature somewhat large dimensions, 
and that the cloud may therefore present to a beholder re- 
garding it from a great distance above, an irregular, pitted, 
notched shape ; in fact, exactly such an appearance as we 
see on the sun, the envelope of which, parvis componere 
magna, may resemble in its mechanism that of a planet 
like our own with its sun withdrawn. 



Circula- 

tion in 

f cuiiating 

gaseous 

masses. 



Eclipse 
pheno- 
mena. 



So far we have only referred to the phenomenon ordi- 
narily visible to us. Another part of the sun's physical 
constitution is rendered visible during total eclipses. We 
allude to the nature of its atmosphere. Eclipse-teachings, 
therefore, arc of high value ; but they certainly are not 
of such high value as ordinary observations of its surface, 
although they are in their nature much more sensational, 
for a total eclipse of the sun is at once one of the 
grandest and most awe-inspiring sights it is possible for 
man to witness. All nature conspires to make it strange 
and unearthly. 



76 



SOLAR PHYSICS. 



CHAH. VI. 



corona 



and 
promt" 
nences. 



Soon the stars burst out ; and surrounding the dark 
moon on all sides is seen a glorious halo, generally of a 
silver-white light : this is called the corona. It is slightly 
radiated in structure, and extends sometimes beyond the 
moon to a distance equal to her diameter. Besides this, 
rays of light, called aigrettes, diverge from the moon's edge, 
and appear to be shining through the light of the corona. 
In some eclipses parts of the corona have reached to a much 
greater distance from the moon's edge than in others. 

It is supposed that the corona is the sun's atmosphere, 
which is not seen when the sun itself is visible, owing to 
the overpowering light of the latter. 

When the totality has commenced, apparently close to 
the edge of the moon, and therefore within the corona, are 
observed fantastically-shaped masses, full lake-red, fading 
into rose-pink, variously called red-flames and red-promi- 
nences. Two of the most remarkable of these hitherto 
noticed were observed in the eclipse of 1851. 

It has been definitely established by the exquisite eclipse 
photographs of De la Rue and Secchi, that these promi- 
nences belong to the sun, as those at first visible on the 
eastern side are gradually obscured by the moon, while 
those on the western are becoming more visible, owing 
to the moon's motion from west to east over the sun. The 
height of some of them above the sun's surface is upwards 
of 70,000 miles. 

The plate on the previous page, of various solar eclipses, 
mai^nUude, may serve to give an idea, at the best a poor one, of 
the extraordinary appearances which are then seen sur- 
rounding the dark moon. Thus, Fig. i represents the 
ordinary phenomena of an annular eclipse ; Fig. 2, the 
annular eclipse of 15th May, 1836, showing Baily's beads ; 
Fig. 3, the total eclipse of 28th July, 1851, as sketched by 
Dawes ; Fig. 4, the total eclipse of 7th September, 1858, 
as observed by Liais ; Fig. 5, the total eclipse of i8th July, 
i860, according to Feilezsch; and lastly, the total eclipse 
of 8th July, 1840. 



Their 



THE SUN A TYPE OF THE MATERIAL UNIVERSE. jj 

It is not yet known what these strange red prominences chap. vi. 
are ; but while we write, astronomers are trooping to India 
to settle the question at the coming total eclipse. England, 
France, Prussia, and other European states will be repre- 
sented, while — a happy evidence of the sooner or later 
prevalence of truth — the successor of Galileo's persecutor 
will be represented by one of the most accomplished 
astronomers of modern times — Father Secchi, a Jesuit, 
who, we trust, will be among the foremost to crown the 
edifice of which Galileo laid the foundation-stone ; ^ for, 
in fact, a knowledge of the nature of the red prominences 
seems now to be the only thing wanting to complete a 
sketch of the visible solar phenomena apart from their 
caif^es. Of course there is much detailed drawing to be 
added afterwards. 

But, even at present, we are in a position to imagine 
what the real nature of the prominences may be. 

In the first place, a diligent spectroscope sweeping round 
the edge of the sun has not revealed any bright lines.* 
This is strong negative evidence that they are not masses 
of incandescent vapour or gas ; for as the light from such 
vapour or gas is almost monochromatic, it should be as easy 
to detect as that of the immeasurably distant nebulae. 

Secondly, we know that the atmosphere of the sun is lite solar 
colder than the photosphere, and that in the latter we "',7^;^^?^' 
have incandescent particles of solid matter. As the pro- than tht 
minences are possibly not due to incandescent vapour, the ^^J 

1 Father Secchi, however, did not go. (1873.) 

« On this point it must be here remarked, that, until the experi- 
ments here referred to were made, it was the settled conviction both 
of Dr. Stewart and myself, that the prominences were masses of glow- 
ing gas. Some of the reasons for this opinion will be found in a 
communication by Dr. Stewart to the Philosophical Magazine in 1862 
(vol. xxiv. p. 305), three years before the experiments were made. 
Others occur upon the surface : for instance, their great actinic power, 
combined with their colour, which could only point to a something 
with lines at both ends of the spectrum ; their shapes and appearances ; 
and their position above the mobile region of the spot level. See my 
communications to the Royal Society in 1866 and 1868, which will 
be given afterwards. (Note added 1873.) 



78 SOLAR PHYSICS, 



CHAP. VI. question remains whether they may be attributable to sub- 
incandescent particles of solid matter at a red glowing 
heat only, suspended in the atmosphere : in fact, whether 
the particles in the photosphere itself may not be likened 
to a white-hot poker, and those in the atmosphere to 
merely a red-hot one. 

In the previous part of this article, attention has been 
directed solely to the immediate cause of a sun-spot ; and 
an attempt has been made to show that a downrush of 
comparatively cold atmosphere from above, accompanied 
with an uprush of warm atmosphere from below, is the 
only sufficient explanation of the phenomena observed. 
It has also been shown, as the result of a careful scrutiny 
of the whole surface of the sun, that there are probably 
convection currents in constant operation all over the disc 
— a condition of things which we might expect from the 
intensely hot state of the sun's surface combined with 
the enormous gravity of matter there placed. A sun-spot 
may thus not improbably be regarded as an enormous 
development under exceptional circumstances of what 
is constantly occurring all over the sun's surface. This 
remark brings us a step further in our inquiry by suggest- 
ing the question. What are the exceptional circumstances 
that cause the ordinary convection currents of the sun's 
surface to develop themselves occasionally into sun-spots.^ 
This inquiry may be rendered more general by dismissing 
from the mind all idea of the nature of sun-spots : it is 
not essential to know what they are, whether convection 
currents or something else. The question now is not what 
is their nature, but what is their cause, or rather, in the 
present state of our ignorance, are they connected with 
any other phenomena that may serve to throw light upon 
their cause .^ This inquiry divides itself into the four 
following heads : — 

I. Does the amount of spotted surface of the sun var>' 
from time to time } 



THE SUN A TYPE OF THE MATERIAL UNIVERSE. 



79 



2. Is the region of outbreak of a spot confined to any 
particular part of the sun's disc ? 

3. When a spot is formed, does it obey any laws with 
regard to increase and diminution ? 

4. And finally, are spots connected with any other 
phenomena on the earth's surface or elsewhere ? 

The remainder of the article will consist of an attempt 
to answer these four questions. 

Now, in the first place, as has been already noticed, the 
amount of spotted surface has a ten-yearly period. This 
has been discovered through the labours of the veteran 
astronomer Hofrath Schwabe, of Dessau, who has now 
for about forty years been engaged without intermission 
in registering the number of spots which appear on the 
sun's surface. 

Herr Schwabe has found as the result of his labours, 
that in the year 1828 there were 225 groups, against 161 
groups in 1827, and 199 in 1829; the year 1828 was 
therefore a year of maximum. After this the number of 
groups gradually decreased until in 1833 there were only 
33 new groups observed. After this year they began again 
to increase, and in 1837 they attained another maximum. 
The next year of maximum was 1848, and the next after 
it 1859. We may therefore expect another in the course 
of a few years ; indeed at the present moment the number 
of spots is increasing. We are still ignorant of the ulti- 
mate cause of this periodicity, but independent observa- 
tions by the Kew Observers,^ and by Hofrath Schwabe, 
lead to the impression that in years of minimum there 
is a less amount of cold-absorbing atmosphere above the 
photosphere, and consequently a smaller tendency to the 
downrush of cold matter in large quantity. The obser- 
vations above referred to seem to indicate for years of 
minimum a more uniform brightness of the sun's surface, 
— that is to say, a less amount of absorption or falling off* 

» "Researches on Solar Physics." ist and 2d Series. By Messrs. 
Ue la Rue, Stewart, and Loewy. 



CHAP. vr. 



period of 
ten years. 



Absorbing 

Qtmosphet e 

less in the 

tninintuhi 

period. 



8o SOLAR PHYSICS. 



CHAP. VI. towards the limb, a phenomenon which, it has been already 
shown, depends upon the amount of cold-absorbing atmo- 
sphere above the region of light. 

We pass on to the second question, as to the r^on of 
outbreak of a spot. 

This question has been answered in an admirable 
manner by Carrington, who showed in a complete dis- 
cussion of all the spots extending from 1854 to i860, that, 
generally speaking, the region of outbreak of spots is the 
spot zones, equatorial zone of the sun. At certain periods, however, 
he has shown that the zone is very closely confined to 
the equator, though at other periods it opens out. Such 
an opening out began about September 1856, at which 
epoch the generality of spots were for the most part found 
at a latitude of 30° either north or south of the solar 
equator. After this they gradually narrowed in towards 
the equator. The date of the next widening out cannot 
be given until the Kew records are reduced, but it is 
believed that at the present moment, or very recently, 
there has been a similar phenomenon. Thus while, gene- 
rally speaking, spots attach themselves to the equatorial 
region of the sun, they are nevertheless inconstant in their 
attachment ; and just as we have a small ripple proceeding 
on the back of a large wave, so we have minor periods 
of opening out proceeding on the back of the large period 
described by Carrington. The Kew Observers have very 
recently described a smaller period of this kind, of four 
months as nearly as possible. 

So much for the solar latitude of sun-spots, and now 
one word with regard to solar longitude. If the sun could 
be sliced like an orange from pole to pole by sections 
of longitude, it is conceivable that one of these sections 
might be found to be composed of a different material 
from the others, more favourable to the development of 
spots. As a matter of fact, however, we have no reason 
for supposing this ; and we believe that the conclusion 
come to by Carrington as the result of his researches is. 



THE SUN A TYPE OF THE MA TERIAL UNIVERSE. 



8i 



that there seems to be no continuous preference given 
to one solar longitude over another as far as regards the 
outbreak of spots. But this leads us on to the next 
question, as to the behaviour of a spot when once formed, 
with regard to increase and diminution. 

Now, while it may with much probability be asserted, 
that no continuous preference is shown to one solar longi- 
tude over another as regards the outbreak of spots, yet 
the longitudinal portion at which a spot breaks out, and 
its behaviour after it has made its appearance, are never- 
theless not accidentally determined. It is an astound- 
ing but apparently well-proved fact, that the birth and 
behaviour of spots are regulated by the position of the 
planetary bodies, so that we may cast the horoscope of a 
sun-spot with some approach to truth. In order to obtain 
grounds for this conclusion, the Kew Observers have labo- 
riously measured the area of all the sun-spots observed by 
Carrington from 1854 to i860, and they find, as the result 
of their inquiries, that a spot has a tendency to break 
out at that portion of the sun which is nearest to the 
planet Venus. As the sun rotates, carrying the newly- 
born spot further away from this planet, the spot grows 
larger, attaining its maximum at the point furthest from 
Venus, and decreasing again on its approaching this planet. 
We here speak of Venus, as it appears to be the most 
influential of all the planets in this respect. Jupiter ap- 
pears also to have much influence; and, more recently, it 
has been shown that Mercury has an influence of the same 
nature, although more difficult to discuss on account of his 
rapid motion. 

Should therefore any two of these planets — or, still 
better, should all three — be acting together at the same 
place upon the sun, we may expect a very large amount of 
spots, which will attain their maximum at that portion of 
the sun most remote from these planets. When we say 
that very good evidence has been shown for this statement, 
we mean that it would have been reckoned conclusive had 





CHAP. vr. 

No pre- 
ferencefor 
any parti- 
cular 
lonj^ude 
far the 
outbreak 
\ of spots. 



Outbreak 
of spots. 



82 



SOLAR PHYSICS. 



CHAP. vr. 



Connection 
of spots 

with terres- 
trial phe- 
nomena. 



Variation 

of smCs 

heat. 



the statement been of a less wonderful character ; and, as 
this conclusion is not less important than wonderful, we trust 
that these researches, which are being prosecuted under the 
auspices of the Royal Society, will be continued until the 
last remnant of doubt is removed from the mind of the 
most sceptical. 

Finally, are spots connected with any other phenomena 
on the earth's surface or elsewhere? For an answer to 
this question we are mainly indebted to the labours of 
General Sabine, the present distinguished President of the 
Royal Society. General Sabine has shown, as the result 
of laborious and long-continued observations in various 
parts of the globe, that there are occasional disturbances 
in the magnetic state of the earth, and that these dis- 
turbances have a periodical variation coinciding in period 
and epoch with the variation in frequency and magnitude 
of the solar spots as observed by Schwabe ; and the 
same philosopher has given us reason to conclude that 
there is a similar coincidence between the outburst of solar 
spots and of the Aurora Borealis. 

Very recently, also, Mr. Baxendell, of Manchester, has 
published some observations from which we may, perhaps, 
conclude that the direct heat of the sun's rays varies 
with the state of the sun's surface. These observations 
require confirmation, but they bear out the idea that at 
these periods there is a greater amount of cold-absorbing 
atmosphere above the sun's photosphere ; that is to say, 
the photosphere is further down or nearer the sun's 
centre, and hence we may suppose of a somewhat higher 
temperature than when it is further up. Under this 
heading it may be stated that we believe Hofrath 
Schwabe conjectures the possibility of a periodicity in the 
appearance of the planet Jupiter, coinciding with the 
period of spot-frequency. This, however, is not yet 
proved.^ 

* We shall subsequently show that terrestrial rainfall and cyclones 
arc intimately connected with sun-spots. 



THE SUN A TYPE OF THE MA TERIAL UNIVERSE, 



83 



We now give the following extract from the conclud- 
ing remarks of the Kew Observers in their paper on 
Planetary Influence: — 

"The following question may occur to our readers: — 
How is it possible that a planet so far from the sun as 
Venus or Jupiter can cause mechanical changes so vast as 
those which sun-spots exhibit? We would reply in the 
following terms to this objection : — 

"We do not of course imagine that we have as yet 
determined the nature of the influence exerted by these 
planets on the sun; but we would nevertheless refer to 
an opinion expressed by Professor Tait, that the properties 
of a body, especially those with respect to heat and light, 
may be influenced by the neighbourhood of a large body. 
Now an influence of this kind would naturally be most 
powerful upon a body such as the sun, which possesses 
a very high temperature, just as a poker thrust into a 
hot furnace will create a greater disturbance of the heat 
than if thrust into a chamber very little hotter than itself. 

The molecular state of the sun, just as that of 

the cannon or of fulminating powder, may be externally 
sensitive to impressions from without, — indeed, we have 
independent grounds for supposing that such is the case. 
We may infer from certain experiments, especially those 
of Caigniard de Latour, that at a very high temperature 
and under a very great pressure the latent heat of vapo- 
rization is very small, so that a comparatively small amount 
of heat will cause a considerable mass of liquid to assume 
the gaseous form, and vice vcrsd. We may thus very 
well suppose that an extremely small withdrawal of heat 
from the sun might cause a copious condensation ; and 
this change of molecular state would, of course, by means 
of altered reflection, &c. alter to a considerable extent the 
distribution over the various particles of the sun's surface 
of an enormous quantity of heat, and great mechanical 
changes might very easily result." 

The speculative outcome of the investigation described 

G 2 



CHAP. VI. 



Effects oj 

small 
increments 
of heat on 
bodies at a 
very high 
tempera- 
ture. 



84 



SOLAR PHYSICS, 



Intimate 
relations 



CHAP. VI. in the latter part of this article may be briefly stated as 
follows : — 

There seems to be great molecular delicacy of construc- 
tion in the sun, and probably also, to an inferior extent, 
in the various planets ; and the bond between the sun 
and the various members of our system appears to be a 
more intimate one than has hitherto been imagined. The 

existing , , . 

behveen the result of all this will bc that a disturbance from witliout is 
^hnett ^^^ easily communicated to our luminary, and that when it 
takes place it communicates a thrill to tlie very extremities of 
the system. 

In a future article the principle of delicacy of con- 
struction will be dwelt upon at greater length, more 
especially with reference to the Place of Life in a Universe 
of Energy. 



THE PLACE OF LIFE IN A UNIVERSE 

OF ENERGY.^ 



There is often a striking likeness between principles which chap. vii. 
nevertheless belong to very different departments of 
knowledge. Each branch of the tree of knowledge bears 
its own precious fruit, and yet there is a unity in this 
variety— a community of type that prevails throughout. 
Nor is this resemblance a merely fanciful one, or one which 
the mind conjures up for its own amusement ; while it has 
produced a very plentiful crop of analogies, allegories, 
parables, and proverbs, not always of the best kind, yet 
parables and proverbs are or ought to be not fictions but 
truths. 

We shall venture to begin this article by instituting an 
analogy between the social and the physical world, in the 
hope that those more familiar with the former than with 
the latter may be led to clearly perceive what is meant by 
the word ENERGY in a strictly physical sense. Energy in Actual 
the social world is well understood. When a man pursues ^'^sy* 
his course undaunted by opposition, unappalled by obsta- 
cles, he is said to be a very energetic man. By his energy, 
we mean the power which he possesses of overcoming 
obstacles ; and the amount of his energy is measured by 
the amount of obstacles which he can overcome, by the 

* A joint communication to Afacmillan^s Magazine by the author 
and Dr. Balfour Stewart, F.R.S., who has permitted me to include 
it here.— J. N. L. 



86 SOLAR PHYSICS, 



CHAP. vir. amount of work which he can do. Such a man may in 
truth be regarded as a social cannon-ball. By means of 
his energy of character he will scatter the ranks of his 
opponents and demolish their ramparts. Nevertheless 
such a man will sometimes be defeated by an opponent 
who does not possess a tithe of his personal energy. Now, 
why is this } The reason is that, although his opponent 
may be deficient in personal energy, yet he may possess 
Energy of more than an equivalent in the high position which he 
posiiton. Qccupies, and it is simply this position that enables him to 
combat successfully with a man of much greater personal 
energy than himself. If two men throw stones at one 
another, one of whom stands on the top of a house and 
the other at the bottom, the man at the top of the house 
has evidently the advantage. 

So in like manner, if two men of equal personal energy 
contend together, the one who has the highest social posi- 
tion has the best chance of succeeding. 

But this high position means energy under another form. 
It means that at some remote period a vast amount of 
personal energy was expended in raising the family into 
this high position. The founder of the family had doubt- 
less greater energy than his fellow-men, and spent it in 
raising himself and his family into a position of advantage. 
The personal element may have long since vanished from 
the family, but it has been transmuted into something else, 
and it enables the present representative to accomplish a 
great deal, owing solely to the high position which he has 
acquired through the efforts of another. We thus see that 
in the social world we have what may be justly called two 
kinds of energy, namely — 

1. Actual or personal energy. 

2. Energy derived from position. 

Let us now turn to the physical world. In this as in the 
social world, it is difficult to ascend. The force of gravity 
may be compared to that force which keeps a man down 
in the world. 



PLACE OF LIFE IN A UNIVERSE OF ENERGY, 87 



If a stone be shot upwards with great velocity, it may be chap. vii. 
said to have in it a great deal of actual energy, because it 
has the power of overcoming the obstacle interposed by 
gravity to its ascent, just as a man of great energy has the 
power of overcoming obstacles. 

This stone as it continues to mount upwards will do so 
with a gradually decreasing velocity, until at the summit of 
its flight all the actual energy with which it started has 
been spent in raising it against the force of gravity to this 
elevated position. It is now moving with no velocity, and 
may be supposed to be caught and lodged upon the top 
of a house. 

Here, then, it rests, without the slightest tendency to 
move, and we naturally inquire. What has become of the 
energy with which it began its flight } Has this energy 
disappeared from the universe without leaving behind it 
any equivalent ? Is it lost for ever, and utterly wasted ? 
Far from it ; the actual energy with which the stone began Energy is 
its flight has no more disappeared from the universe of c/tanga/in 
energy than the carbon which we burn in our fire dis- /<"w- 
appears from the universe of matter. 

It has only changed its form and disappeared as energy 
of actual motion in gaining for the stone a position of 
advantage with respect to the force of gravity. 

Thus it is seen that during the upward flight of the 
stone its energy of actual motion has gradually become 
changed into energy of position, and the reverse will take 
place during its downward flight, if we now suppose it 
dislodged from the top of the house. In this latter case 
the energy of position with which it begins its downward 
flight is gradually converted into energy of actual motion, 
until at last, when it once more reaches the ground, it 
has the same amount of velocity, and therefore of actual 
energy, which it had at first. 

Thus we have also in the physical world two kinds of 
energy : in the first place we have that of actual motion, 
and in the next we have that of position. We see from 



88 SOLAR PHYSICS. 

CHAP. VII. this how intimate is the analogy between the social and 
the physical worlds as regards energy, the only difference 
being that, while in the former it is impossible to measure 
energy with exactness, in the latter we can gauge it with 
the utmost precision, for it means the power of performing 
work, and work (it is needless to mention in this mechani- 
cal age) is capable of very accurate measurement. 

There are several varieties of energy in the universe, and, 
Proteus-like, it is always changing its form. Had it not 
been for this habit, we should have understood it long 
since, but it was only when its endeavours to escape from 
the grasp of the experimentalist were of no avail, that 
it ceased its struggles and told us the truth. 
Forms of All of these varieties may, however, be embraced under 
'"^^^' the two heads already mentioned, — namely, energy of 
actual motion and energy of position, 

A railway train, a meteor, a mountain torrent, represent 
Energy of energy of motion^ but there is also invisible molecular 
niouou. niotion which does not the less exist because it is invisible. 
Such for example is heat, for we have reason to believe 
that the particles of hot bodies are in very violent motion. 
A ray of light is another example of energy of motion, and 
so likewise is a current of electricity ; and if we associate 
the latter with a flash of lightning, it ought to be remem- 
bered that the flash is due to particles of air that have 
been intensely heated by electricity becoming changed 
into heat. Electricity in motion is pre-eminently a silent 
energy, and it is only when changed into something else 
that its character becomes violent. 
Eftergy of Then, again, as representing energy of position we may 
posuton, instance our stone at the top of the house, or a head of 
water, both of which derive their energy from their advan- 
tageous position with respect to gravity. 

But there are other forces besides gravity. Thus a 
watch newly wound up is in a condition of visible advan- 
tage with respect to the force of the main-spring, and as it 
continues to go it gradually loses this energy of position, 



PLACE OF LIFE IN A UNIVERSE OF ENERGY, 89 

converting it into energy of motion. A cross-bow bent is chap. vn. 
likewise in a pK)sition of advantage with respect to the 
spring of the bow ; and when its bolt is discharged, this 
energy of pK)sition is converted into energy of motion. 

Besides this, there are invisible forms of energy of Imnsibu 
pK)sition. When we tear asunder a stone from the earth, f^'^*- 
and lodge the former on the top of a house, we obtain 
visible energy of position, the force against which we act 
being gravity. But we may also tear asunder from each 
other the component atoms of some chemical compound, 
our act here being performed against the very powerful 
force called clumical affinity. 

Thus, taking a particle of carbonic acid, we may tear 
asunder the oxygen from the carbon, and, if our scale of 
operations be sufficiently great, we shall obtain separate 
from each other one mass of carbon and another of oxygen, 
— not, however, without the expenditure of a very large 
amount of energy in producing this separation. 

We have, however, obtained a convenient form of energy 
of jx)sition as the result of our labours, which we may 
keep in store for any length of time, and finally, by allow- 
ing the carbon and oxygen to reunite, — that is to say, by 
burning the carbon, — we may recover in the shape of heat 
and light the energy which we originally expended in 
forcing these bodies asunder. 

Some of the most prominent varieties of energy of 
motion and of position have now been described, and the 
remarks made have induced the belief that this thing, 
energy, this capacity which exists in matter for performing 
work of one kind or another, is by no means a fluctuating 
element of our universe, but has a reality and a perma- 
nence comparable to that which we associate with an atom 
of matter. 

The grand principle of the conservation of energy, a Consen'a- 
principle lately proved by Dr. Joule,* asserts that energy, ^^. 

> We ought not to omit the names of W. R. Grove and Mayer in 
connection with this generalization. 



90 SOLAR PHYSICS. 



CHAP. VII. like ordinary matter, is incapable of being either created 
or destroyed. We will endeavour to give two examples 
in illustration of this great law, which is worthy of the 
highest attention. 
Rumford Let US first ask, with Rumford and Davy, When a 
and Davy, hammer has struck an anvil, what becomes of the energy 
of the blow } or when a railway train in motion has been 
stopped by the brake, what becomes of the energy of the 
train } A proper understanding of what here takes place 
will very much conduce to a clear conception of the laws 
of energy. 

Unquestionably in both these instances energy seems to 
have disappeared — to have vanished, at least, from that 
category which embraces visible energy, and we arc taught 
to ask if the disappearance means annihilation or only a 
change of form. Let us examine what other phenomena 
accompany this seeming disappearance. It is well known 
that an anvil or piece of metal repeatedly struck by a 
hammer becomes hot, nay, even red hot, if the process be 
continued long enough. It is also known that when a 
railway train is stopped there is much friction at the brake- 
wheel, from which on a dark night sparks may be seen to 
issue. We may add to these the experiment of Davy, in 
which two pieces of ice arc melted by being rubbed against 
each other. The concomitants of percussion and friction 
are thus seen to be in the first place an apparent destruction 
of energy, and in the second the apparent generation of 
heat ; and this mere juxtaposition of the two phenomena 
is quite enough to suggest that in this case mechanical 
energy is changed into heat. 

The second example to be mentioned in illustration of 
the laws of energy is the origin of coal or wood. Coal or 
wood, as we all know, is a very concentrated and convenient 
form of energy. We can bring a great deal of heat out of 
it, or we can make it do a great deal of mechanical work. 

Now as wood grows, from whence does the wood derive 
its energy } We are entitled to ask this just as fairly as 



PLACE OF UFE IN A UNIVERSE OF ENERGY, 91 



from what source it derives its particles. The wood, we chap. vh. 
answer, derives its energy from the sun's rays. Part of 
these rays is spent in decomposing carbonic acid in the 
leaves of plants, ejecting the oxygen (one of the products 
of this decomposition) into the air, but retaining the carbon 
in the leaf, and ultimately building up the woody fibre 
from this very carbon. 

Nothing for nothing in these regions. The sun's energy Nothing 
is spent in producing the wood or coal, and the energy of ^^ ^^/. ^ 
the wood or coal is spent (far from economically, it is to 
be regretted) in warming our houses and in driving our 
engines. 

These two illustrations will tend to impress upon the 
minds of our readers the truth of the grand principle of 
the conservation of energy. 

The principle now described has reference, however, Dissipa- 
merely to quantity, and asserts that in all the various ^'^" ^ 

^ ^ *" energy. 

transmutations of energy there is no such thing as creation 
or annihilation. An additional principle discovered by Sir 
W. Thomson, and named by him the "dissipation of 
energy," refers to quality. And here also there is a striking 
analogy between the social and the physical world ; for as 
in the social world there are forms of energy conducing to 
no useful result, so likewise in the physical world there 
are degraded forms of energy from which we can derive 
no benefit. And as in the social world a man may degrade 
his energy, so also in the physical world may energy be 
degraded ; in both worlds, when degradation is once 
accomplished, a complete recovery would appear to be 
impossible, unless energy of a superior form be commu- 
nicated from without. 

The best representative of superior energy is mechanical 
effect. Another is heat of high temperature, or the means 
of producing this in the shape of fuel. 

The mechanical energy of a machine in motion may 
not only give us useful work, but, if we choose, we can 
transmute it cither directly or indirectly into all other 



92 SOLAR PHYSICS. 



CHAP. VII. forms of energy. Again, high-temperature heat is another 
very useful form of energy, and by means of the steam- 
engine it may be converted into mechanical effect. On the 
other hand, when heat is equally diffused or spread about, 
it represents the most degraded and worthless of all forms 
of energy. Nothing of value can be accomplished by its 
means. Thus, for instance, there is abundance of heat 
spread throughout the walls of the chamber in which we 
now write, but not a particle of all this can be converted 
into useful mechanical effect. 

Long before any of these laws were known the supe- 
riority of certain kinds of energy was instinctively recog- 
nized ; and desperate, but of course futile, efforts have 
ever and anon been made by enthusiastic visionaries to 
procure a perpetual motion or an ever-burning light We 
could amuse our readers, if we had time, with some of 
these : the lesson they teach is that no ingenuity can raise 
a superstructure without foundations. The possibility of 
a perpetual motion still lingers in the minds of certain 
enthusiasts, but the idea of an ever-burning light has 
vanished long since ; it seems more than the other to 
have been associated with pretensions to magic. Thus, 
in ''The Lay of the Last Minstrel," we find the monk 
of St. Mary's Aisle describing in the following words the 
grave of the famous wizard Michael Scott : — 

" Lo, warrior ! now the cross of red 
Points lo the grave of the mighty dead ; 
Within it burns a wondrous light. 
To chase the spirits that love the night. 
That lamp shall burn unquenchably, 
Until the eternal doom shall be." 

Now the law of the dissipation of energy shows us at 
once why a perpetual motion and an ever-burning light 
are both equally impossible. It asserts that there is a 
tendency in the universe to change the superior kinds of 
energy into inferior or degraded kinds, which latter can 
only to a very small extent be changed back again into 
superior forms. Thus we have seen how easy it is by 



PLACE OF LIFE IN A UNIVERSE OF ENERGY. 93 



percussion or friction to transmute all the mechanical chap. vu. 

energy of a blow or visible motion into heat, but only a 

very small portion of this heat can be transmuted back 

into visible motion. There is, in fact, a tendency abroad 

to change all kinds of enei^y into low-temperature heat 

equally spread about, — a thing that is of no possible use 

to anyone. 

Seeing, then, that our existence and well-being depend 
on the presence in the universe of a large quantity of 
superior energy, which we may be able to utilize, it be- 
comes us to look about us^ and take stock as it were 
of the goods that have been placed at our disposal. Now 
the nearest approach to an ever-burning lamp is the sun, 
and a near approach to a perpetual motion is represented 
by the motion of the earth on its axis, and it will shortly 
appear that it is from these two sources of superior energy 
that we draw all our supplies of this indispensable com- 
modity. 

Of the two sources the sun is by far the most impor- Importnnre 
tant. Let us examine very briefly the extent of our 9f ^*'**' 
obligations to our great luminary. In the first place, with- 
out his energy in the shape of heat and light everything 
in the world would be frozen and dark ; for the little 
heat left, being unrecruited, would very soon pass off into 
space, and our scanty stock of fuel would form a very poor 
substitute for the sun's rays. But this is only a small part 
of what we get from the sun, for we have already hinted 
that it is by means of the energy of his rays as absorbed 
by the leaves of plants that carbonic acid is decomposed, 
and coal and wood produced, coal being a product of the 
past and wood of the present age. 

Food has the same origin as fuel ; it is in fact the fuel Food fuel. 
which we burn in our own bodies instead of on our hearths 
or in our engines. Without a proper supply of food we 
should soon cease firstly to perform work and ultimately 
to live, and the more hard work we have to accomplish the 
more food must be taken. 



94 SOLAR PHYSICS. 



CHAP. VII. In like manner, without a proper supply of fuel a steam- 
engine would soon cease to perform work. Again, wind 
and water power, or the power of air and water in motion, 
ought not to be forgotten as forms of energy which may 
be usefully applied. These also are indirectly due to our 
luminary, whose heat produces currents in the atmosphere, 
and also carries up in the form of vapour the waters of the 
ocean to be again precipitated in the form of rain. Wind- 
mills and watermills are therefore due to the sun as well 
as steam-power and muscular energy. Tidal energy 
stands, however, on another footing. The tides are pro- 
duced by the action of the moon and of the sun upon 
the waters of the ocean, but the energy which they re- 
present is not derived from these luminaries, but from the 
rotative energy of our own globe, which is gradually losing 
its speed of rotation from this cause, although at a rate 
which is extremely small, indeed almost infinitesimal. 

Is it then the case that we have been furnished on a 
grand scale with that which enthusiasts have in vain tried 
to imitate on a small one, namely, — an ever-burning light 
and a perpetual motion } 

If we allow that myriads of years bear a nearer approach 
to eternity than a few hours, then we may assert that this 
is the case ; but if we regard all duration and all magni- 
tude as comparative, then we have only been furnished 
on a large scale as regards both these elements with what 
we can ourselves produce on a small one. 

The principle of degradation is at work throughout the 
universe, not less surely, but only more slowly, than when 
it combats our puny efforts, and it will ultimately render, 
it may be, the whole universe, but more assuredly that 
portion of it with which we are connected, unfit for the 
habitation of beings like ourselves. As far as we are 
able to judge, the life of the universe will come to an end 
not less certainly, but only more slowly, than the life of 
him who pens these lines or of those who read them. 

It is desirable to state clearly, and once for all, that our 



PLACE OF LIFE IN A UNIVERSE OF ENERGY. 95 



standpoint in what follows is that of students of physical chap. vii. 
science. We are here only as such students, and, from 
the trifling elevation which we may have reached as 
followers of science, we shall endeavour to answer, it may 
be imperfectly, but yet honestly, certain questions which 
might be put to us by those who are interested in knowing 
" how the day goes." 

More particularly then with regard to the place of life, 
— What are the conditions necessary in order that the 
universe may be a fit abode for living beings ? 

It has already been shown that one of these conditions Deikacy of 
is the existence in the universe of a quantity of energy, not ^^j^" 
in a thoroughly degraded state, but capable of producing 
useful effect ; we have now to add that another condition is 
i/ie capability of great delicacy of organization. 

The motion of the universe would seem to be of two 
kinds ; it is in fact the old story of a shield with two sides, 
each side with its champion, and the quarrel between 
them very hot. If we reflect, we shall see that the per- 
fection of the laws which regulate the larger masses of 
the universe, such as planets, consists in the fact that the 
motions produced are eminently capable of being made the 
subject of calculation. But, on the other hand, the very 
perfection of the animated beings of the universe consists 
in the fact that their motions cannot possibly be made 
the subject of calculation. A man who could predict 
his own motion is an inconceivable monster ; in fact, 
having calculated what he is about to do, he has only 
to do the opposite in order to show the absurdity of the 
hypothesis. 

This freedom which is given to animated beings is 
nevertheless held quite in conformity with, and in sub- 
jection to, the laws of energy already mentioned, but it 
requires as a condition of its existence great delicacy of 
organisation. 

In order to comprehend what is meant by this expres- 
sion, we may imagine to ourselves a universe consisting 



96 SOLAR PHYSICS. 



CHAP. VII. of nothing but carbon and oxygen separate from one 
another. Such a universe would possess to a very large 
extent a superior kind of energy, yet we cannot by any 
possibility imagine how such materials could be moulded 
into organized forms or become the residence of living 
beings. The very idea of its sable monsters provokes 
a smile, although we might perhaps be at a loss were 
we asked definitely to state our objection to this condition 
of things. 

Let us, however, consider this imaginary universe for 
a moment, and the nature of its deficiency will soon 
appear. If on fire, it will continue to burn at a rate which 
may be calculated without much trouble ; if not on fire, 
it will continue as it is. There is not, therefore, in such 
a universe any, or hardly any, capacity for producing 
or sustaining delicate organizations possessing freedom 
of motion. 

A living being (at least one of a superior order) is 
not only a machine capable of producing motion, but 
of producing it discontinuously, and in a great variety of 
ways which cannot be calculated upon except to a very 
limited extent. 

In this respect there is a class of machines analogous to 
some extent to living bodies. Suppose, for instance, a gun 
loaded with powder and ball, and very delicately poised ; 
then by the expenditure of a very small amount of energy 
upon the trigger, a stupendous mechanical result may be 
achieved, which may be greatly varied : touch the trigger, 
and the gun is discharged, driving out the ball with great 
velocity. The direction of its path will, however, depend 
upon the pointing of the gun ; if well pointed, it may ex- 
plode a magazine, — nay, even win an empire. 

Here, then, there is a very stupendous result in the way 
of visible motion produced through tjie agency of a very 
small amount of energy bestowed upon the trigger, and 
all in conformity with the conservation of energy, since 
it is a certain kind of energy of position resident in the 



PLACE OF LIFE L\ A VXJJ'EKSE OF EXEi^GT 



gunpowder dial has been ciiangrd into iDrrhaniral cfiect : cea? vc 
but yet the result cannot be adneved viiiK>Qt tiie applica- 
tion of this small amount of dircdiTe cna^ to the tii^er, 
for if the tr^^ger be touched too lig^itly the gxm will not 
go off The small amount of cpeig ;> bestosrcd upon the 
tr^S^r beoMnes, as it vcre. the parent or source of ^le 
much laiger amount of energy of the cannon-balL We 
have in (act here a madune of gremt though nnzi£ delicac}' 
of oxistruction. 

It is not, howei'er, imposable to suppose a machine of 
infinite ddicacy of ctmstrMCtum. We may, for instance, 
imag^e an electric arrangement so drlirare that by an 
amount of directive energ}* less than any assignable qism- 
tity a current may be made to start suddenhr. cn>s vot^ 
Atlantic, and (as far as pfa^^scal results are concerned ex- 
plode a magazine on the other side. Indeed, the fooxes of 
nature appear to be such that an in&iite delicacy of con- 
struction is not inccMiceri'able. 

We have thus considered two cases of madiines having: 
great delicacy of constructiazL In the former of these it 
required a certain finite and definite amount of energy to 
be exf)ended on the trigger before the gun -was discharged. 
but in the second case things were brought to such a pas? 
that by an application of an amount of ener^' less than 
any assignable quantity, the electric circuit v ou'id be ren- 
dered complete. The first case in fact represents^ a madiint 
of great but yet finite delicac>- : the latter, a niachrne of 
infinite delicacy of construction. 



Let us now proceed to state the \-arious concei^*able f unc- Fm»t<t • 
lions that life may be supposed to discharge witii relation ^ ""'' 
to the energy of the universe : we say concd\-able, for in 
the sequel the reader \i-ill be called c»n to select from a list 
of four kinds of action, of which two, although conodi-abie, 
are yet extremely improbable. Our choice therefore must 
finally be restricted to two conceptions, neither of which 
is inconceivable or impossible as far as the laws of ener-^y 

H 



r 



98 



SOLAR PHYSICS. 



CHAP. VII. 



Matfrialis 
tic vim: 



Second 
hypothe />. 



are concerned ; and between these two we must finally 
choose on other grounds than can with propriety be treated 
of in this article. 

There are four functions which life or intelligence may 
be supposed to discharge. In the first place, there is the 
purely materialistic view of life, which may be stated thus: — 
A living being is a very complicated machine^ consist- 
ing of matter very delicately organized, but containing 
besides no other principle ; so that, if we knew completely 
the laws of matter and the position of the various par- 
ticles which constitute the machine, and if we knew 
at the same moment the disposition of the exterior 
universe which is capable of influencing the machine, and 
if our methods of calculation were sufficiently developed, 
we should be able to predict all future motions of the 
living being. 

The second hypothesis is, that life or intelligence has the 
capacity for creating energy. This view is so very impro- 
bable that we may dismiss it with a very few remarks. 
What we can say with truth is that, in all experiments 
and observations which we have been able to examine 
thoroughly, energy is not created. It is conceivable that 
there may be a region beyond our ken in which enei^ is 
created, but, arguing according to the principles which are 
universally admitted to be our guides in such matters, wc 
must pronounce the creation of energy by a living being to 
be out of the question.^ 

The third conceivable hypothesis regarding the function 
hypothesis, ^f jjf^j j<; ^j^^^^ which asserts that life, although it cannot 

create energy, can yet transmute immediately, and by virtue 
of its presaice, a finite quantity of energy from one form to 
another. It is necessary to explain the meaning of the 
word immediately. Referring to the gun with a delicate 
trigger, which we have already alluded to, it cannot be 
said that the immediate cause of the motion of the ball 
was the energy bestowed upon the trigger: the immediate 
' This was recognized at an early period by Carpenter and JoutC. 



Third 



PLACE OF LIFE IN A UNIVERSE OF ENERGY. 99 



cause of this motion was the aeriform state which the gun- chap. vh. 
jx)wder had assumed, while again the immediate cause of 
the change of state in the gunpowder was the heat de- 
veloped by the explosion of the fulminating powder in the 
touch-hole, and the cause of the powder's exploding was 
the blow given to it by the hammer of the lock. The blow 
again may be traced to the action of the lock-spring, which 
is set free to act through the small impulse communicated 
to the trigger. We see from this, that whenever a finite 
amount of energy changes its form, — as, for instance, when 
the chemical energy of the gunpowder is changed into 
the mechanical energy of the ball, — we naturally look to 
some material circumstance which precedes and explains 
this change. We may be quite certain that the gunpowder 
will not explode unless a small quantity of high-tempera- 
ture heat be communicated to it, nor will the fulminating 
powder explode unless it receives the blow, nor will the 
blow be given unless the trigger is pulled. 

Thus, in this example, if we are able to change some 
energy which we have at hand into visible energy sufficient 
to pull the trigger, that small change will form the original 
germ of the much greater one implied in the explosion 
of the powder and the motion of the ball, or rather it 
will be the first link in a series of changes of which the 
last is the motion of the ball ; and so in similar machines 
we find a change of energy preceded by some other change, 
perhaps much smaller in amount, which explains it. And 
now the question arises, Can life, while it does not create 
energy, be yet the immediate cause of the change of a 
finite quantity of energy from one form to another, which 
change would not have taken place without the presence 
of life, and which is not, therefore, preceded by a material 
cause in the shape of a parent change of energy } We 
cannot readily allow that life can act thus, for this would 
imply that of the finite and measurable changes of energy 
which take place in the universe, and which therefore either 
are, or may become, subjects of experiment and observa- 

H 2 



loo SOLAR PHYSICS, 

CHAP. VII. tion, some are immediately preceded by a material 
cause, and some by an immaterial one, and that this is 
the regular system of things : to the minds of most men 
an uncertainty of this nature in the immediate causes of 
measurable results will appear improbable d priori^ and, 
moreover, it is a view entirely unsupported by experiment 
and observation. Let us, therefore, dismiss this view of the 
action of life, and consider the only other view of its 
action which appears to be possible. 
Fourth Assuming, therefore, that life can neither create enei^ 

hypothesis. ^^^ ^^^ immediately transform a finite amount of energy 
from one form to another, may not the living being be 
an organization of infinite delicacy, by means of which a 
principle in its essence distinct from matter, by impressing 
upon it an infinitely small amount of directive energy, may 
bring about perceptible results? We have shown that 
such a class of machine is conceivable, when we suggested 
a certain electrical arrangement, and we know that our 
bodies are machines of exquisite delicacy. Such a mode 
of action of the vital principle is not, therefore, inconceiv- 
able, and, by supposing that it does not immediately change 
a finite quantity of energy from one form to another, wc 
get rid of that element of irregularity which we cannot 
easily admit to be consistent with the order of nature. 

We come We are thus presented with two hyjx)theses of the action 

kypo^es ^^ ^^^^' ^^^ ^^^^ ^^ these is the materialistic hypothesis, 
which denies the existence of life as a principle apart from 
matter ; while the other allows the existence of an inde- 
pendent principle, but assumes its action to take place 
through the medium of a machine of infinite delicacy, so 
that by a primordial impulse of less than any assignable 
amount a finite and visible outcome is produced. These 
are the two alternatives, and it is not within our province 
to attempt to decide between them. The battle must be 
fought in other pages than ours, and by other weapons 
than those which we can produce. 



PLACE OF LIFE IN A UNIVERSE OF ENERGY. loi 

Let us here pause for a moment to consider the won- chap. vii. 
derful principle of delicacy which appears to pervade the PHndpu 
universe of life. We see how from an exceedingly small ofddkacy, 
primordial impulse great and visible results are produced. 
In the mysterious brain chamber of the solitary student we 
conceive some obscure transmutation of enei^y. Light is, 
however, thrown upon one of the laws of nature; the 
transcendent power of steam as a motive agent has, let us 
imagine, been grasped by the human mind. Presently the 
scene widens, and, as we proceed, a solitary engine is seen 
to be performing, and in a laborious way converting heat 
into work ; we proceed further and further until the pros- 
pect expands into a scene of glorious triumph, and the 
imperceptible streamlet of thought that rose so obscurely 
has swelled into a mighty river, on which all the projects of 
humanity are embarked. 

And now a hint to those who are disposed to adopt that 
theory of life which demands an infinite delicacy of con- 
struction. 

May it not be possible that in certain states of excite- is tiure 
ment there is action at a distance } This is a field of inquiry dloarue" 
which men of science do not seem disposed to enter, and 
the consequence is that it appears to be given over to 
impostors. We need scarcely, after this, inform the reader 
that we do not believe in so-called spiritual manifestations ; 
nevertheless we ask, does there not appear to be an amount 
of floating evidence for impressions derived from a distance 
in a way that we cannot explain ? For are not the most 
curious and inexplicable actions of instinct those in which 
distance seems to be set at nought ? Then, again, if we 
take the element time, instead of distance: — who has 
not felt some past scenes, perhaps of his early childhood, 
called up suddenly and vividly before him by some 
trivial sight, or sound, or smell? May there not, after 
all, be a deep physical meaning in these words of the 
poet : — 



ii.m to the 
sun. 



I02 SOLAR PHYSICS. 

CHAP. VII. " Yet still, from time to time, vague and forlorn, 

From the soul's subterranean depth upborne, 

As from an infinitely distant land 

Come airs and floating echoes, and convey 

A melancholy unto all our day." 

Hitherto we have been confining our thoughts to the 
realms of life, in which the principle of delicacy is suffi- 
ciently obvious, but the results of a preceding article will 
have prepared our readers for a wider application of this 
\ppiica- principle. It is not only in the organic world that we see 
a delicacy of construction, but in the inorganic also. Thus 
it will be remembered that, in discussing the molecular 
state of the sun, we came. to the conclusion that it was one 
of great delicacy, so that in our luminary a very small 
cause might be the parent of enormous effects, of a visible 
and mechanical nature. And when we came to analyse 
the behaviour of sun-spots, we found that this behaviour 
had a manifest relation to the positions of the two planets 
Venus and Jupiter, although these two planets are never 
so near the sun as they are to our own Earth. We have 
also shown that sun-spots or solar disturbances appear to 
be accompanied by disturbances of the earth's magnetism, 
and these again by auroral displays. Besides this, we have 
some reason to suppose a connection between sun-spots 
and the meteorology of our globe. From all these circum- 
stances we cannot fail to remark that the different members 
of our system (and the thought may be extended to other 
systems) are more closely bound together than has been 
hitherto supposed. Mutual relations of a mathematical 
nature we were aware of before, but the connection seems 
to be much more intimate than this — they feel, they throb 
together, they are pervaded by a principle of delicacy even 
as we are ourselves. 

We remark, in conclusion, that something of this kind 
might be expected if we suppose that a Supreme Intelli- 
gence, without interfering with the ordinary laws of matter, 
pervades the universe, exercising a directive energy capable 
of comparison with that which is exercised by a living 



PLACE OF LIFE IN A UNIVERSE OF ENERGY. 103 

being. In both cases delicacy of construction would chap. vii. 
appear to be the thing required for an action of this 
nature. 

Bearing in mind, however, our physical standpoint, we 
cannot venture to offer any further remark on this subject. 
Whether such a mode of action is a fact must be decided 
by other considerations ; whether it would appear to be 
physically possible is a question which we may suppose 
put to us, and which we have ventured to answer as 
above. 



THE PLACE IN SCIENCE OF THE NEW 

METHOD, 

CHAP. VIM. The year that has just passed from us^ will be for ever 
memorable in the history of science. It has given to 
astronomers a method of studying the physics of our 
central luminary, the sun, at once so delicate and so search- 
ing that a new era of observation has dawned ; it has, 
moreover, seen the settlement of a question which has 
puzzled mankind for a century and a half, and put us 
in possession of new facts of the highest interest and 
importance. 

The sun shining in unclouded and uneclipsed splendour 
is a sight familiar to us all, and, time out of mind, mankind 
has watched his risings and settings, and poets have sung 
of the panoply of cloud and glory in which he then appears. 
In these ordinary aspects, however, the sun for the last 
150 years has been invested with comparatively little in- 
terest to the astronomer ; it is only when hid in dire eclipse 
that the feeble human eye can appreciate all the wonders 
of our great light-giver — and total eclipses happen but 
very rarely. 

Whenever the sun shines, the brilliant envelope, called 
the photosphere, is full of wondrous teachings ; and the 
marvellous sun-spots, the discovery of which sent a shock 
as of an earthquake through the minds of the Schoolmen 

» This was written in 1868. 



THE NEW METHOD, 105 



at the beginning of the seventeenth century, are among the chap.viii. 
most striking and stupendous effects of force which it is 
given to man to witness. But these are now but ordinary 
phenomena ; we are familiar with them ; and we are apt 
to forget the scale on which the changes rendered visible 
to us by our telescopes take place. This is not the case 
with the different classes of actions which, though for 
ever going on, are only visible to us when, during eclipses, 
the moon shields our eye, and, by interposing herself 
exactly between us and the sun, allows us to inspect the 
sun's atmosphere with perfect ease. Then new glories are 
rendered visible, which make the moments of the totality 
as precious to scientific men as they are terrible and awe- 
inspiring to ordinary beholders. One seems in a new 
world — a world filled with awful sights and strange fore- Phmometia 
bodings, and in which stillness and sadness reign supreme j ^^ ^^ -'• 
the voice of man and the cries of animals are hushed ; the 
clouds are full of threatenings and put on unearthly hues : 
dusky livid, or purple, or yellowish crimson tones chase 
each other over the sky irrespective of the clouds. The 
very sea is responsive, and turns lurid red. All at once 
the moon's shadow comes sweeping over air and earth 
and sky with frightful speed. Men look at each other 
and behold, as it were, corpses, and the sun's light is 
lost. 

And then.^ Then the astronomer at his telescope sees Thewoik 
the edge of the moon, which hangs like a black ball in ^{ronlnur, 
the heavens, suddenly, as if at the call of a magician, 
bedeckt here and there with strange-looking tongues of red 
flame, and at those parts where the edges of the sun and 
moon most nearly fit each other lower ridges are seen, 
lying close to the moon's edge — or limb — and continuous 
for some distance. As the moon travels over the sun from 
west to east, these strange " red flames," still maintaining 
their shapes, vary their size, and then, just before totality 
is over, another line of ridges is seen on the opposite part 
of the moon to that on which they first appeared. Then 



io6 SOLAR PHYSICS. 

CHAP. VIII. the sun appears again, and these strange things are lost to 
human eye till another total eclipse conies round with 
its glorious revelations. 

Red flames. The discoveries to which we have now to draw attention 
refer to the nature of the " red flames," " prominences," or 
** protuberances," as they have been variously called, visible, 
as we have seen, during total eclipses of the sun ; but to 
discuss these discoveries at all usefully it is necessary that 
we should trace up our knowledge of the red flames them- 
selves, and state the optical principles on which the dis- 
coveries are based. 

Grmvth of First, then, as to the growth of our knowledge concerning 

ourkfmi- ^Y^Q ^gj flames. As far as I know, the first mention of 

leage. ' 

those strangely beautiful and weird appendages of the 
sun, variously called "red flames," "prominences," and 
'* protuberances," which are visible, and only visible, during 
a total eclipse of our great luminary, occurs in a letter 
stannyan. addressed by a Captain Stannyan to Flamsteed, in 1706; 
that is, 162 years ago. Stannyan was at Berne, observing 
the total solar eclipse of that year, when the sun was 
totally darkened for four minutes and a half; he seems 
to have had sufficient presence of mind to have given 
the marvellous and awful accompanying phenomena only 
their due share of attention ; for he carefully watched for 
the sun's reappearance, and was rewarded by observing 
that "his getting out of his eclipse was preceded by a 
blood-red streak of light from his left limb, which con- 
tinued not longer than six or seven seconds of time ; then 
part of the sun's disc appeared all of a sudden as bright 
as Venus was ever seen in the night — nay brighter ; and 
in that very instant gave a light and shadow to things 
as strong as the moon uses to do." It seems pretty clear 
that Stannyan believed this " blood-red streak " to belong 
to the sun, for he does not mention the moon ; but unfor- 
tunately authority, in the shape of Flamsteed, referred it 
without question to the moon ; and the height of our satel- 
lite's atmosphere was at once calculated to a nicety. 



THE NEW METHOD, 107 

A similar phenomenon was recorded by Halley and chap.viii. 
Louville in 171 5 — the former describing " a long and very Halley aud 
narrow streak of a dusky but strong red light," which -^^^'^-^<'- 
"seemed to colour the dark edge of the moon;" the 
latter seeing " an arc of a deep red colour." These, doubt- 
less, were the equivalents of the " streak " seen by Stannyan, 
and were not "^prominences " properly so called. The 
theory that these strange things belonged to the moon 
and not to the sun was not, however, shaken by these 
observations ; and when Vassenius described to the Royal Vassenit.s. 
Society the eclipse of 1733, he boldly placed the "reddish 
spots" which he observed " in the lunar atmosphere." It 
may be safely stated that until the year 1842 the idea 
that the red flames were entities in, or appearances caused 
by, the moon's atmosphere, was never called in question ; 
and it was not banished from men's minds till the year 
i860. It held its own, therefore, for over a century and 
a half — a pretty long run for an assertion made on such 
slender basis, but one not altogether unprecedented. 

From 1706 to i860, total eclipses of the sun have swept 
over Europe. I believe that in every case — certainly in 
every late case — the remarkable phenomena first observed 
in 1706 have been seen. In astronomical observation as 
in other matters, Ce nest que le premier pas qui cofite 
— the mind helps the eye as well as the eye the mind ; 
but the records are singularly unsatisfactory till the year 
1842 is reached, and then the golden age begins. The Eclipse oj 
*' red flames " in that year were watched by several observers * ^' 
of the highest eminence. It is the first eclipse, in fact, 
of which we have full and scientifically accurate obser- 
vations ; as a consequence, our knowledge of the rose- 
coloured prominences was largely increased. They were 
measured with the utmost care, and their various colours 
and general appearances were recorded. Mauvais com- 
pared two of the prominences in shape and colour to the 
peaks of the Alps illuminated by the setting sun. Mr. Airy 
likened them to saw-teeth in the position proper for a 



io8 SOLAR PHYSICS. 

CHAP. VIII. circular saw. Mr. Baily describes those seen by himself 
encircling the black moon as follows : — ^They had the 
appearance of mountains of a prodigious elevation ; their 
colour was red, tinged with lilac or purple; perhaps the 
colour of the peach-blossom would more nearly represent 
it. They somewhat resembled the snowy tops of the 
" Alpine mountains when coloured by the rising or settiQg 
sun." One observer poetically remarked that if the lumi- 
nous points he saw had extended all round the moon it 
would have resembled "a box of ebony garnished with 
rubies." Littrow saw them change from white to red 
and from red to violet, and then back again through the 
reverse order. 

But the most important observations made during this 
Mauvais eclipse have yet to be referred to. Mauvais, a French 
'^obsSva^ astronomer who was stationed at Perpignan, saw a reddish 
tions. point transform itself into two large protuberances, and 
soon a third began to be visible to the left of thero. 
While the third " mountain " appeared to be issuing forth, 
the first two observed continued to increase. Further, 
Petit, another French astronomer, also saw them rapidly 
increase in magnitude as if they were uncovered in conse- 
quence of the motion of the moon — in fact, they seemed to 
emerge from the eclipse pari passim with the sun. Here 
then, apparently, was evidence that they belonged after all 
to the sun, and not to the moon. 
Eclitseof The next swing of the eclipse pendulum brings us 
* ^** to 1851, and to Sweden, which was in consequence the 
rendezvous of European, but especially of English, as- 
tronomers, who were now convinced, particularly by the 
observations of 1842, of the enormous interest and import- 
ance of the problem. Of this eclipse we have admirable 
records. Airy, Adams, Dawes, Hind, Carringfton, Robin- 
son, Dunkin, Lassell, were among the eminent observers 
who were there to endeavour to settle the question. Pro- 
minences there were in abundance, some of them of g^^at 
magnitude and striking form. So enormous in height 



THE NEW METHOD, 



109 



and so brilliant was one of them, that it was clearly visible chap.vih. 
to the naked eye, and Dawes saw it five seconds after 
the sun had reappeared ! We owe to that lamented 
observer the most minute account of the prominences. 
One of them was cone-shaped, of a deep red colour ; 
another was a bluntly triangular pink body, disconnected 
with the sun ; and another like a " Turkish cimeter," 70,000 
miles in height if it belonged to the sun, with one of its 
edges of a rich carmine colour. Besides these, two low 
ridges were seen stretching along the moon's edge ; in one 
of them was a flame like a "dog's tusk," its colour and 
brilliancy varying from those of the lower ridges. The 
prominence which reminded Dawes of a "cimeter" was 
likened by Airy to a " boomerang," its colour *' full lake 
red ;" the latter also saw one of the ridges, which he called 
a " sierra," situated along the sun's edge at the part where 
it was just fitted by the edge of the moon; this sierra 
being more brilliant than the other prominences, and its 
colour scarlet. This eclipse left a very distinct impression 
on the Astronomer Royal's mind as to the exact place 
of the prominences. It was impossible, he said, to see 
the changes that took place in the prominences without 
feeling the conviction that they belonged to the **sun 
and not to the moon." Still Professor Adams was not 
quite convinced. Mr. Dunkin held a contrary opinion to 
Airy ; other observers, if they had formed one, did not 
express it ; and we believe that the general consensus will 
be faithfully represented by sa}ing that at this period, 
the whereabouts of the prominences — i.e, whether they 
belonged to the sun or moon — was ** not proven." There 
was strong evidence going to show that they could only 
belong to the sun, but the theory was not thoroughly 
established. 

Still the eclipse of 1851 was not without its results. 
The prominences lent themselves admirably to minute 
observations, the work done in former years was endorsed, 
and the old mountain-shaped prominences and the low 



Verdict, 
pnn'en, " 



SOLAR PHYSICS. 



ciiAP.vnr. sierras, of a different colour from the prominences proper, 

and more brilliant, were again observed.^ 
EcUfistuf The next attack was made in iS6o. the astronomical 
' forces having in the meantime secured an ally of tre- 




mendous power. Hy this time celestial photography, in 
the hands of Mr. Warren De la Rue, had arrived at a high 
state of perfection. And now the prominences were pkcto- 

I Since the articles were wrliten 1 h.ive learni thai the name of Mr. 
Swan should have been added, for among the moat impartant resull) 
of this ectijise are three papers communicaled by him lo the Ro)-jt 
Society of Edinbut^h, in wnlch he clcnrly points out, as Grant had 
done for the observations before 1851, that tne prominences are pan 
of the sun, and (Mat tktre is a conlinHOUs stratum 0/ promiiumt 
maiter surrounding the sun. 




THE NE W METHOD. 1 1 r 



graphed, not only by Mr. De la Rue himself, but by Father chap.viii. 
Secchi, who has followed in the wake of all work of this 
nature. The prominences now told their own story on his 
photographic plates, and announced themselves as belong- 
ing beyond all question to the sun. It is impossible to 
speak too highly of the skill with which the aid of this new 
method of recording astronomical phenomena was invoked 
and utilized by Mr. De la Rue, in spite of the probability 
that his work would all be useless. He writes : — " The 
general impression I formed from the information derived 
was that the light emitted by the corona " (a halo of dim 
white light which surrounds the moon during total eclipses) 
" and red flames, taken together, was about equal to that 
of a full moon — less rather than greater ; but no one 
recollected precisely the brightness of the prominences as 
compared with that of the corona. With this imperfect 
information as a guide, an attempt was made at Kew 
to photograph the moon, but not the slightest impression 
could be procured of our satellite by an exposure of the 
sensitive plate during one whole minute to its image in the 
heliograph. My expectation of success in getting pictures 
of the totality was not great after this trial ; nevertheless, 
I still thought it desirable to carry on the experiment to 
the end, on account of the value of the results, if I should 
fortunately succeed." 

Mr. De la Rue was able to obtain the sun's own evidence Photogin- 
of the famous Spanish eclipse in an almost unbroken series P^*^^^''^'*^^^- 
of upwards of forty photographs, from the time the moon 
made her first appearance on the sun till the time she had 
entirely crossed it. Just before the sun was totally hid, the 
prominences became visible in the telescope, and were re- 
corded on the photographic plate ; a long line of low ridges 
being visible when the eastern edge of the moon, which was 
travelling from west to east, was coincident with the just 
hidden edge of the sun. Tops of high prominences were also 
registered where the moon (which appeared much larger than 
the sun) extended grossly beyond the sun's edge, especially 



SOLAR PHYSICS. 

I. the western one. Just before the sun began to reappear on 
the opposite side, and when the western edge of the moon 
nearly fitted the still hidden western edge of the sun, 
another low sierra appeared at the western edge, l/ie one 
formerly observed being by this time covered up by tlu moon 
The accompanying woodcuts contain examples of these 
changes, taken from the Indian eclipse of l868. 




Nothing could be more complete than the proof thui; 
afforded that these appendages belonged to the sun: the 
prominences were eclipsed and uncovered exactly as the 
sun itself was ; their whereabouts, therefore, could no 
longer be questioned ; and if, as I shall show presently, 
this fact was not established up to and including 184;, to 




THE NEW METHOD. 

Mr. De la Rue belongs the full credit of having solved this ( 
important question, which had remained ittb judice for a 
century and a half. Here, then, at length was the great 
question solved in a manner that admitted of no doubt. 
The prominences told their own story ; they at last yielded 
up their secret They were fairly run to earth. 

Mr. De la Rue remarked, moreover, that on comparing 
the results of the expedition of i860 with those obtained 




/ auloTloulur. PrDmincnca ihDwn in Fiu ijii.xr 

^' aIniMi tonteiy iwrnakd. while mlim (Dmiirly 



m rSsi.the general similarity of the prominences at the 
two epochs was very striking :—" On both occasions were 
^ccB luminous masses of vast extent, perfectly detached 
from the sun and far beyond the lunar disc; the same 
"regularity of outline on the convex side running out into 
points, the same apparent outpouring of faint vapours 
falling, as it were, towards the sun." Mr. De la Rue was 
lot content with his own photographs. He made a 




Crmftr. 



. careltil oimparison of them with those taken by Father 
Sccchi, who observed the eclipse at some distance from 
"/fL- his station ; and he found important differences in them — 
itgra/*). exactly such differences, too, as must have arisen from the 
difference of position of the observers if the prominences 
realty belonged to the sun. To thoroughly understand 
this, let the reader slide a shilling representing the moon 
over a sixpence representing the sun, from right to left, ir. 
west to east. 




It was distinctly evident that the elevation of the 
prominences above the moon's northern limb was much 
higher in Mr. De la Rue's pictures than in Father Secchi's ; 
a fact accounted for by the moon having been seen much 
higher at Desierta (Father Secchi's station) than at Riva- 
bcllosa, where Mr. De la Rue was. Similarly, the promi- 
nences seen beyond the moon's southern limb were most 
uncovered in Secchi's photographs. 



THE NEW METHOD. 1 1 5 

In the year i860, then, was settled the question as to the chap.vih 
position of the strange things first observed in 1706, and . 

the settlement of this point had brought with it large towfure- 
additions to our knowledge concerning their shapes, sizes, '^^^^^^ ?f 
colours, and the like. nences 

One of the prominences roistered by Mr. De la Rue, ^^^^'^ 
for instance, extended 72,000 miles from the sun's surface 
into its atmosphere ; and there was also evidence that the 
substance of which some of them were composed gave 
out light much stronger chemically tJian visually^ for tJiey 
allowed themselves to be registered on the pJiotographic 
plate while they were invisible to the eye. These and 
innumerable other points of interest, to which we cannot 
do more than refer, lent enormous importance and emphasis 
to the next question that was asked of these strange 
things. 

*• What are they ? " was on the lips of all interested in ^^j^ ^^^ 
science; But how to tell what they were ? How to solve the promt- 
this final riddle ? Telescope, camera, and eye had done ^'^^^ ' 
their utmost, but their all was unavailing in such an inquiry 
as this. The astronomers were baffled ; they not only 
knew they were ignorant, but they saw no way out of their 
difficultiM — there was no other line of research open to 
them. 

In science, however, as in other things, help sometimes spectrum 
comes from a very unexpected quarter. It was so in this analysis, 
case. Before even the Himalaya had started with its 
freight of astronomers to view the famous Spanish eclipse, 
two men had been working independently, one in Heidel- 
berg and the other in Edinburgh, on the subject of radiant 
light and heat, and the final result of the labours of these 
physicists, Kirchhoff and Balfour Stewart, was that at last 
the world of science was put in possession of just such a 
method of research as we have shown it coveted in order 
to settle the nature of the red flames. 

The researches of Kirchhoff showed, in fact, that the 
light proceeding from any substance contains, as it were, 

I 2 



1 1 6 SOLAR PHYSICS. 



MiAP.viii. an autobiography of that substance — in a strange lan- 
guage, certainly, but one capable of being translated into 
the vulgar tongue by passing a light through a prism. 
If, for instance, we look at the .flame of a candle through 
a common lustre, we find the yellow — almost white — light 
which it gives out transformed into a broad, rainbow- 
coloured band ; a continuous riband of coloured light, red 
at one end and passing through yellow, green, blue, and 
indigo to lavender at the other. If, with certain pre- 
cautions which need not be stated here, we examine the 
light given out by a gas, or by the vapour of burning 
substances, such as sodium, iron, &c., we find this con- 
tinuously lighted-up ' riband replaced by bright lines, 
scattered here and there along the space that would be 
covered by the band of coloured light if we were looking 
at a candle. In the fact that these bright lines vary with 
every substance which we can examine we have the basis 
of Spcctrinn Analysis. 

ConftiY/ion But SO far we have said nothing of the connection of this 
discovery with the sun. The sun's spectrum is not like 
either of those previously referred to. The light is not 
continuous, and it does not consist of bright lines. It 
is coloured exactly as it would be if it w^ere continuous, 
like the spectrum of a candle, but crossing it at right 
angles to its length, are innumerable black lines or bars 
of greater or less thickness. Kirchhoff* — and this is the 
great merit of his discovery — explained these lines. He 
pointed out, first of all, that many of them occupied 
exactly the same position in the spectrum as the bright 
lines given out by such substances as sodium, iron, &c, do; 
and he next showed that it was possible to reproduce such 
a spectrum at will, by allowing a bright light to shine 
through the vapour of sodium, iron, &c., in which case the 
cool vapour uses up the light of the same colour that 
it itself would emit, and the otherwise continuous spectrum 
of the bright light reaches the eye with a bar across it. 
We may sum up Kirchhoff^s results as follows : — 



:i7//; t/ii 
sun. 



THE NEW METHOD. 



117 



1. When solid or liquid bodies are in a state of incan- 
descence they give out continuous spectra. 

2. When solid or liquid bodies, reduced to a state of gas, 
or any gas itself burns at ordinary pressure, the spectrum 
given out consists of bright lines only, and these bright 
lines are differetit for different substances, 

3. When light from a solid or liquid incandescent body 
passes through a gas, the gas absorbs those particular rays 
of light of which its own spectrum consists. 

By carefully mapping the lines in the solar spectrum, and 
by matching them with the bright lines given out by gases 
and the vapours of solid substances, Kirchhoff established 
beyond all question that sodium, iron, magnesium, barium, 
copper, zinc, calcium, chromium, nickel, and aluminium 
were present in the sun*s atmosphere : to these hydrogen 
has since been added. 

The reader should now be able to see the drift of what we 
have stated with regard to the application of this method 
of research to the red flames. An analysis of their light 
must at once give us information as to their solid or gaseous 
nature. If the light they emitted gave a continuous spec- 
trum, then we should say they were either solid or liquid. 
If, on the contrary, their spectrum consisted of bright 
lines, then we should know that they were gaseous. In the 
latter case the particular gas or vapour would be deter- 
mined by the position of the bright lines in the spectrum. 

This problem was attacked in 1866, and without waiting 
for an eclipse! 

I had been carefully investigating sun-spots both with 
telescope and spectroscope (an instrument which consists 
of carefully-arranged prisms), and had succeeded in show- 
ing by means of the latter marvellous means of research 
that they were due to a greater absorption of the solar 
atmosphere in certain localities ; and having had the benefit 
of several conversations with my eminent friend Dr. Balfour 
Stewart, the conclusion we arrived at was that the red 
flames were probably masses of incandescent gas. 



CHAP.Vin, 

T/if main 
points of 
spfcfrum 
analysis. 



Elements 
in sun. 



New 
attack. 



1 1 8 SOLAR PHYSICS, 



cHAF.viii. On this hypothesis it became at once obvious that their 
existence should be revealed by the spectroscope without 
the occurrence of a total eclipse, as they are not then 
rendered visible by any magical or mysterious process, but 
simply by the absence of the overpowering light of the sun : 
for although the red flames are only visible to the eye when 
the sun is eclipsed, it does not follow that their existence 
will not be detected by tJie spectroscope at other times ; 
and for this reason, — the prominences are not visible to the 
eye in ordinary sunshine, because the regions near the sun 
are as bright or brighter than the prominences ; they are, 
therefore, " put out," as the stars are in daytime. But mark 
what will happen if they really be built up of gas, and 
their light in the spectrum is concentrated into a few bright 
bands. The light of the sun's atmosphere, which is made 
to enter the instrument through an excessively narrow slit, 
will be spread out into a long band ; the light will cover 
a large area and will become diluted in consequence. But 
the light from the prominences (coming through the same 
slit) will, on the contrary, scarcely be spread out at all, it 
will remain concentrated in two or three or more lines, and 
in the spectroscope the prominence lines should for this 
reason be seen as bright lines on Xht fainter background of 
the spectrum of the atmosphere. I began to act up>on this 
liuiurc. idea in i866; but the only result of my efforts was to show 
me that the means at my disposal were not sufficient to 
attack the problem with any chance of success. It was 
essential that I should obtain the spectrum of the edge of 
the sun and the regions just outside it, and that the latter 
should be dark enough to form a background for the 
bright linos that would be seen here and there projecting 
from the .solar spectrum if the hypothesis that the pro- 
minences w^ere gaseous was correct. In my instrument, 
ReasjHs howcvcr, the illumination of the sun's atmosphere by the 

for failure, y^^^^ reflected from the outer bright shell of the sun itself 
— the photosphere — and the illumination of our atipo- 
sphere especially were so great near the sun, that the 



THE NE W ME THOD. i ; 9 



background was not dark enough to allow bright lines to chap.mii 
be easily visible, and I failed to detect any lines, though 
I diligently ** Ashed" round the sun\s limb many times, 
and in all probability passed over prominences. I there- 
fore communicated my idea to the Royal Society, and my 
difficulties to the Government Grant Committee. The 
matter was thought worthy of their aid, and in the be- 
ginning of 1867 an instrument was being constructed, which 
owing to a chapter of accidents I only received incom- 
plete on the 1 6th of October, 1868. Owing to these delays 
in the construction of this instrument, the solution of the 
question was left to the eclipse of 1868, visible in India. 

Nearly all the principal scientific bodies of Europe took jttjtan 
up the matter with great warmth. Our own Royal Society ^apse^ 
and the Royal Astronomical Society both sent out expe- 
ditions, and so did the Academy of Sciences and the 
Bureau des Longitudes. Prussia and Austria were also 
represented. The eclipse was to happen on the i8th of 
August, and the results were anticipated with the utmost 
impatience and interest. The result of the observations 
in India was decisive as to the nature of the prominences. 
The spectroscope settled this as satisfactorily as the camera 
had settled their whereabouts in i860. At last the tele- 
grams came. The two words "bright lines," were quite ''Bfigit 
sufficient to tell the scientific world that one large part of ^""' 
the problem had been settled. The "red flames" were 
really built up of glowing gas or vapour. All the observers 
had seen those tell-tale lines during the eclipse which had 
in vain been looked for in the full glory of the sun with 
my small instrument. One lai^e part of the final question 
was for ever put to rest The prominences were built up 
of incandescent gas or vapour. 

But what gas, or what vapour } This would be indicated Om qu^s- 
by the relative positions of the bright lines referred to the ^'^'^ ^^^ 
solar spectrum itself— that glorious band of rainbow hue, 
from red through orange, yellow, green, blue, indigo, and 
lavender, crossed at right angles to its length by innu- 



SOLAR PHYSICS. 



. mcrable black lines, which is the very cypher of the uni- 
verse, but which nevertheless has been read a little. With 
which of the black lines did these newly-discovered and 




all-eloquent bright lines coincide? Here the eclipse gave 
out an uncertain sound. Indue course of time we received 
detailed accounts from three of the expeditions, under 
Captain Herschcl, Major Tennant, and M. Stcphan re- 




THE NEW METHOD. I a 

spectively, and representing the Royal Society, the Royal chap. 
Astronomical Society, and the Bureau dcs Longitudes. 
In the detailed accounts, although it was clear that every 
man had tried his utmost, there was not such great subject 
of congratulation as there had been in the telegrams. 
There was still one part of the question unsettled. All 
the observers had observed bright lines, but they were not 




certain as to the positions of some of the lines, and the 
accounts were discordant among themselves. Premising 
that the principal black lines in the solar spectrum are 
lettered from the red towards the lavender end, the follow- 
ing results will be readily understood. Captain Herschel 
wrote : — 

" I consider that there can be no question that the orange 
line was identical with D, so far as the instrument is com- 




SOLAJi PHYSICS. 






petent to establish an identity. I also consider that the 
identity of the blue line with F is not established, but, on 
the contrary, I believe the former is less refracted than F, 
but not much. With regard to the red line, I hesitate very 
much in assigning an approximate place. It might have 
been near C ; I doubt its beingas far as B, but there would 
be the limit. I am not prepared to hazard any more de- 
finite opinion about it. Its colour was a bright red," 




Major Tennant saw five lines, three of them corre- 
sponding to C, D, and b, one in the green near F, and 
possibly one near G. 

M. Rayet (in M. Stephan's party) saw nine lines, corre- 
sponding to B, D, K, b, F, and the others undetermined. 

It is obvious from these discrepancies, which wc hold 
to be entirely unavoidable in such a delicate investigation. 




carried on under difliculties of weather and under con- * 
ditions so out of the common, that the question as to the 
nature of the red Rames — our knowledge of which depended 
upon a rigorous determination of the position of the hnes 
— was left open ; and it seemed very much as if, after all, 
the problem would be left in uncertainty and doubt until 







['» phuuigrapha. (Aug 




. another total eclipse with its attendant phenomena swept 
L over the earth. A total eclipse of the sun is an awful 
J phenomenon, and it was scarcely to be expected that in 
I its presence a tremendous problem should be solved at 
tlhe very first attempt. All the observers found all their 
|Wx)rious preparations of many months culminating in a 



THE NEW METHOD. 125 

few minutes, and those minutes rendered part almost of a chaf.vin. 

new existence in a new world by the unaccustomed look 

of things. The mental tension must have been extreme : 

the hope of widening the range of knowledge, and the 

fear of losing a single precious instant, are not calculated 

to steady either the hand or the eye, and it is no discredit 

to these men to point out that the results they obtained 

were discordant as to the positions of the bright lines 

observed. 

Such was, as I imagined, the condition of things when, Suatss. 
on the 20th October, four days after I had received my 
new instrument — which promised to succeed if any trust 
was to be placed in principles — I at last saw for the first 
time the long- wished -for lines ; and, in my observatory, the 
sun shining in all the glory that an English autumn permits, 
at my leisure, though not without excitement, measured 
their absolute positions on the solar spectrum itself, both 
the bright lines proceeding from the prominences and the 
brilliantly coloured cypher-band proceeding from the sun's 
edge being spread out before me, allowing an absolute 
means of determining the position of the former with 
reference to the latter, all doubt, uncertainty, or error as 
to their position being rendered impossible — an advantage 
which the eclipse observers were deprived of, owing to the 
temporary obscuration of the sun. 

Three beautifully coloured lines of light were visible. i.ittc> 
Two of them corresponding to C and F in the solar spec- 
trum showed that the famous red flames were composed in 
part, at least, of incandescent hydrogen gas; that hydro- 
gen gas was present in the atmosphere of the sun in 
volumes beside which the size of the earth is as nothing, 
welling up in what may be almost considered tongues of 
flame to a height of 70,000 and 80,000 miles, now running 
out into strange shapes and branches, now parting from 
the lower surface and floating cloudlike in the higher 
regions. Besides these two lines, which settled the question 



r/.w/'A 



126 



SOLAR PHYSICS. 



CHAP. VIII. as to hydrogen, another h'ne was observed near D, which 
strangely enough, had no dark line in the solar spectrum, 
corresponding with it. I soon found that by sweeping the 
slit of the spectroscope along the sun's edge and over the 
prominences, it was quite easy to determine their outline, 
the length of the bright line visible giving the height of 
that part of the prominence on the slit at the time. 
jiiHssen. Here, then, was all doubt and uncertainty removed as 
to the position of the lines, and a method discovered of 
mapping the prominences every day the sun shines, instead 
of glimpsing them every ten years or so. Immediately after 
a second letter giving some further details had been com- 
municated to the President of the Academy of Sciences in 
Paris, a letter was received from M. Janssen, one of the obser- 
vers of the eclipse, to the effect that the same idea which 
I had published in 1866, had struck him during the eclipse 
itself ; that he had applied it the next day, and had deter- 
mined the absolute position of the lines at his leisure, as I 
did. The fact, which was determined by both of us, that 
bright lines corresponding to C and F in the solar spectrum 
appeared in the spectrum of the prominences, as before 
stated, settled the question as to their nature, for these are 
the two principal lines given out by hydrogen gas. The 
spectroscope, therefore, had taught us that the prominences 
were composed wholly, or in part, of hydrogen — incan- 
descent hydrogen gas, bursting up in tongues of flame and 
cloudlike masses from the photosphere. 

M. Janssen continued to observe the prominences for 
seventeen days after the eclipse, and before his observa- 
tions were received it was known that they could not fail 
to be of the highest importance, for he is a practised ob- 
server, has long devoted himself to spectroscopic research, 
and had a sun not far from the zenith to work upon,^ 

^ As this article appeared in the Times, a paragraph was inserted 
here on the question of priority. I omit it from the text, but in justice 
to M. Faye I introduce it as a note : — 

" Here, then, was a nice point of priority to settle. Mr. Lockyer had 
been the first to conceive the plan and to announce its success, but M 



THE NEW METHOD, 127 

The eagerness with which the details of M. Janssen's chap.viii. 
seventeen days' work in India were waited for may be 
imagined. In the meantime I had continued to apply the -^J^^^^/"/ 
new method ; the few observations possible during the vor^. 
months of October and November had opened out a 
broad expanse of new fields of investigation and specula- 
tion which a few months ago were undreamt of. Before, 
however, we pass to this later work, it will be well to refer 
to the details of M. Janssen's observations, which arrived 
in Paris in due time. 

M. Janssen's detailed account of his work, consisting of a 
report to the President of the Bureau des Longitudes, is of 
very high interest. During the eclipse he does not appear 
to have been more fortunate than the members of the other 
expeditions ; he is uncertain even as to the number of lines 
observed, whether five or six, but the uncertainty was not 
of long duration. M. Janssen was struck with the great 
brilliancy of the red flames, and the idea occurred to him 
that — we use his own words — " it might be possible to see 
them without an eclipse. Unfortunately, the weather, which 

Janssen had made the discovery really two months before Mr. Lockyer, 
one month of the time being taken up in the transmission of the news 
from India. No doubt we should have heard very much more of this 
singular coincidence had not M. Faye, one of the most distinguished 
astronomical members of the Academy, at once attempted a solution 
of the question of priority in a manner which reflects the highest 
credit on him for his even-handed justice. He pointed out that, 
although M. Janssen had really been the first to succeed, Mr. Lockyer 
had first suggested the idea in 1866, and also that Mr. Lockyer's prior 
announcement depended really upon M. Janssen's being in India. 
He did not forget, also, to indicate that Mr. Lockyer's idea had re- 
mained long without fruit, and that not only had the obser\ers of the 
eclipse not given any attention to it, but that the English observers 
had omitted to make any attempts to see the red flames after the 
eclipse had happened. He concluded by remarking: — * Instead, 
therefore, of endeavouring to apportion, and therefore to weaken the 
merit of the discovery, is it not better to attribute the whole honour, 
without any reservation, to both of these men of science, who, sepa- 
rated by some thousands of miles, have each been fortunate enough 
to reach the intangible and the invisible by the method the most 
astonishing, probably, that the genius of observation has ever con- 
ceived.^"* 



128 SOLAR PHYSICS. 



cHAP.viif. clouded over after the last contact, did not allow me to 
make any experiment that day. During the night the 
method and the means of observation were settled in my 
mind. On the morrow, the 19th, having got up at three, I 
made the necessary arrangements for the new observations. 
The sun rose without a cloud, and as soon as it was free 
from the low mists on the horizon I commenced work." 

At last, precisely as it happened to me, a beautiful line 
flashed up at C, then the existence of another at F was 
determined. The line at D was not observed by M. Janssen. 
Having determined these bright lines, M. Janssen's atten- 
tion seems to have been entirely devoted to the indications, 
afforded by the varying brilliancy and length of the lines, 
of the intense action going on, and he declares as a result 
of his investigations that the prominences ** are the seats 
of movements of which no terrestrial phenomena can give 
any idea, masses of matter many hundred times larger 
than the earth changing both place and form in the space 
of a few minutes." 

Later We now come finally to my later work, which branches 

off curiously enough from M. Janssen's after the discovery 
and determination of the prominence spectrum. 

Between the 20th October and the 5th November, my 
spectroscope had been rendered more complete (modern 
science is, alas ! very dependent upon complete instru- 
ments), and its next revelation startled me as much as the 
first one had done. The beautiful tell-tale lines added to 
their former story : not only were the prominences proved 
ConHmt' beyond all question to be hydrogen, but the fact, that tlicy were 

'/ >/>''^ ^ ;//^vr/^' local hcapiugs up of a hydrogen envelope which attirely 
surrounded the sun %vas established. The examination of 
light from all parts of the sun's edge showed that outside 
the photosphere the prominence spectrum was never absent, 
and I may add that since the day named, except once 
in a dense fog, it has never been absent from the field 
of view of my instrument, whenever I have looked at the 



THE NEW METHOD, 129 

sun — which, thanks to our terrible climate, has happened cjiai'.vii:. 
at intervals, alas ! few and far bet>veen. This envelope ~ 
has been named the chromosphere, to distinguish it from 
the atmosphere on the one hand, and the white photo- 
sphere on the other. 

And here, before I go further, a retrospect is neces- Retrosf^ect. 
sary. When I commenced my observations, I had no 
idea that it had ever been suggested that the promi- 
nences were part of a continuous envelope. After I had 
established the existence of this envelope, an examination 
of Mr. De la Rue's admirable photographs, and of other 
records, led me to believe that it had really been indicated 
over and over again, though the indications had been 
neglected. I have lately, however, been referred by Mr. 
De la Rue to a report by M. Le Verrier, which I had 
not previously seen, on the eclipse of i860, in which the 
idea of a continuous envelope is distinctly enunciated ; 
and since I have begun this article I have found that such 
an idea was suggested by Professor Grant before the eclipse 
of 1851, from a most complete analysis of all the observa- 
tions made up to that time, and reported in his admirable 
"History of Physical Astronomy."^ It is true that Mr. 
Grant does not refer this third envelope to what we now 
know to be the right cause, but to him undoubtedly 
belongs, as far as I now know, the credit of having sug- 
gested that the prominences might be merely a part of 
such an envelope, while I have shown they really are 
As I have confessed my own prior ignorance of Mr. Grant's 
masterly analysis of this matter, I may be permitted to 
express my surprise that it had been so generally over- 
looked ; so far as I am aware, such an idea was never 
broached either in connection with the eclipse of i860 or 
1 868 ; whereas, had it been, the continuity of the enve- 
lope might have been established easily by observations 
at properly chosen stations, quite independently of the 
spectroscope. 

* I> 395 401. 

K 



1 30 SOLA R PHYSICS. 



cHAr.viii. In the same chapter Mr. Grant shows also that the 
early eclipses afford ample evidence that the prominences 
belong to the sun, although, as we have seen, this fact 
was not considered to be definitely settled till i860. 
To the hydrogen envelope, the existence of which, as an 
envelope, has now been established by means of the 
spectroscope, I have, at the suggestion of my friend 
Dr, Sharpey, given the name of Chromosphere, as it is 
the region in which all the various and beautiful coloured 
phenomena are seen. 

Here at last, then, is the veil somewhat uplifted — 

who shall dare to say how little.^ Under it we see 

the meaning of the "blood-red streak" observed 162 

years ago by Stannyan at Berne, — a meaning finally 

revealed to us by a process which renders the invisible 

sensible to the human eye ; which allows us, as it were, 

to feel from world to world. And is this the end ? No ; 

the veil is still being uplifted, for modern science moves 

apace. Though *' Ars longa, vita brevis," is, alas ! still too 

true, its truth is not the old truth ; it is now becoming a 

question more of extent than of time. The wondrous 

cypher-band has other secrets to reveal, and it seems 

already as if we were about to dwarf our prior efforts to 

dive into the secrets of the sun. The spectrum is, in fact, 

a link which binds worlds so closely together, that every 

terrestrial laboratory is an observatory; and, per contra, 

the sun may teach us chemistry. 

At the beginning of my observations, the behaviour of 
one of the new bright lines was so strange and unexpected, 
that I was for a time completely puzzled ; its message was 
hard to read, but an alteration in the instrument made 
the matter clearer. The hydrogen spectrum at the upper 
niveau of the chromosphere was different from the spec- 
trum of the lower level, — precious indications, going far 
to prove that with patient research we may not only in- 
crease our knowledge of the hydrogen spectrum by obser- 
vations of the prominences, but may arrive at a knowledge 



THE NEW METHOD. 131 



of the temperature and density of these circumsolar chap.vih . 
regions. 

It is not a little singular that a method so clearly in- 
dicated in 1866 should have waited till 1868 for its ulti- 
mate success, and the coincidence in time of my results 
with the receipt of the detailed news from India might 
lead one not in possession of all the facts to imagine that 
the success really depended upon the eclipse observations.^ 
Now, however, that we have the method, it may be 
safely affirmed that astronomers have, as it were, taken 
a new lease of the sun, and have work before them for 
many years to come. And as we have seen telescope 
give way to camera, and camera to spectroscope, a time 
will doubtless come when the spectroscope itself will give 
place to some other yet undreamt-of instrument, with which 
our children or children's children will investigate the 
glorious problem of the sun. 

The story of the recent work must now for a time be 
interrupted in order that the fundamental points of the 
new science of Spectrum Analysis, on which so much of 
it has been based, may be placed as tersely as may be 
before the reader. 

* Sec the discussion of this point further on. 



K 2 



THE BIRTH OF SPECTRUM ANALYSIS. 



Thk field of research which we are now about to con- 
sider is one which has been opened up so recently that 
it may be said that the spectroscope is to the men of 
science of to-day what the telescope was to Galileo and 
Fabricius. For even at the present time, although im- 
mense strides liave been made during the last decade. 




Fir., jt, -Gem 



the science of spectroscopy must still be considered 
in its infancy. And yet, so far as one can see now — 
it is always very easy to prophesy after the event — there 
seems very little reason why the recent discoveries should 



THE BIRTH OF SPECTRUM ANALYSIS. 

not have been made years ago ; for nearly two centuries t 
have elapsed since the immortal Newton, following up the 
researches of Kepler and others, made his classical re- 
searches on the action of a prism upon sunlight. 

In dealing with the application of spectrum analysis to 
solar physics, the first part of the story must neces- 
sarily in the main consist of an account of the prism and 




the principles of the spectroscope, and then of a descrip- 
tion of the various kinds of spectroscopes which are now 
employed in solar research; the applications of the spec- 
troscope with regard to terrestrial matters must also be 
touched upon, so that the full force of the method can 
be grasped when we come to consider its application to 
the sun. 

Obviously, the first question we have to answer is this, 
What is a spectroscope ? This is answered by saying that 
a spectroscope is an instrument in which the action of a 



SOLAR PHYSICS. 



prism or a combination of prisms is rendered most efTectual. 
The next question, then, that arises, is. What is a prism ? 
Figs. 31 and 32 will give a good idea of what is meant by 
a prism, and little time need be spent in description. Let 
us rather come to its action. 

If a beam of sunlight be allowed to enter a dark roooa 
from a round hole in a shutter, it will travel in a straight 
line from its source; and to make it deviate from 1 
straight line, one of two things must be done. The beam 
must either be reficcUd or refracted. 

First, as to reflection. The reflection of light ii of v 
ordinary occurrence, for when light strikes any polis 



with parol- 
Msur/arr,. 





Lighi puiipi ihrough i plaie of stos. 



metallic surface, or in fact a surface of any kind, it is more 
or less reflected by it. The phenomena of rcflectioa aie 
so well known, the use of the mirror or looking-glnss baqg 
perhaps one of the most tangible, that no detailed referenoe 
need be made to them. 

Let us next interpose in the path of the ray of light a 
piece of t^lass, such for instance as window glass, t/ie sur- 
faccs of tchic/i arc parallel (Fig, 33). Whether the glass 
is inclined lo the beam or not, it will be observed that 
the direction of the beam through the room will not be 
changed : the reason of this is, that when we get the light 
falling on the glass from the air, then travelling through 
the glass, and coming into the air again, under exactly 
the same conditions, what is done at the first surface— and 



THE BIRTH OF SPECTRUM ANALYSIS. 135 

something is done, as we shall see presently— is exactly chap. 1 

undone at the second, so that we get pretty much the „ ~ 

same effect as at first. But noiv. if instead of having the „J ^1 

glass bounded by parallel surfaces, we use a /m;«. the «'^'"<'" 

sides of which are not paralltl, you will see that there is p,„a!i,. 




a distinct alteration in the effect produced ; the beam 
is now forced to another portion of the wall allugether. 
The ray strikes the first side of the prism, and is bent 
towards the thicker part, or towards a line perpendicular 
to the surface on which it strikes ; and on reaching the 
second side ot the wedge, the ray is again bent in the same 



1 36 SOLAR PHYSICS. 



CHAP. IX. direction, towards the base of the prism ; in this case the 
ray is bent away from the perpendicular to the second 
surface, and the light emerges from the second surface 
in a totally new direction. An experiment may easily be 
tried, which will confirm this. Let a prism of glass be 
held, with an edge upwards, between the eye and a lighted 
candle, as shown in Fig. 34 ; it will be found that the candle 
cannot be seen ; but if the prism be gradually raised, the 
image of the candle will appear, the aimount the prism 
will have to be raised depending on its angle. Now, we 
have here obtained a deination or refraction of light, — that 
is to say, it has been bent out of its course ; for wc 
have to look upwards to see the candle. 
Kepler's Now this effect and something more had been obsen'cd 

"^Pilma^c ^y Kepler, who thus refers to it in his " Dioptrics : " » 

refraction. , 

** A vioma Scnsualc, 

" XVI. 

"Colores Iridis jucundissimi oriuntur cum refractio est 
tanta : idquc tarn si oculi transpiciant quam si Sol trans- 
luceat. 

** XVII. 

** Sole prisma irradiante tria genera radiorum resultant, 
Sincerus, Vitri colore, et Iridis coloribus. 

"Sit enim F Sol. Is radiet in D. Hie quasi dividitur 
radii Solaris densitas, quae minima sui parte repercutitur in 
D I, ct anguli, A I) I, equali ipsi B D F quo illabitur. Sincerum 
igitur radium, scd tenuem per D I vibrat in I. Sincerus est. 
quia in vitro tinctus non est, cujus corpus non ingreditur. 

" Potior autem pars de densitate ipsius F I) penetrat I> 
ct rcfringitur in D E. In E vero rursum dividitur, rationc 
densitatis. Potior enim pars transit E, et propter geminam 
magnam rcfractionem colores Iridis jaculatur in G. 

'* Residuum ipsius D E tenue admodum repercutitur a 
supcrficie A C in E M : quod si D E paulo obliquius in A K 

» ** Dioptricc, sivc demonstratio corum quae visui ct visibilibus prop- 
ter Conspicilla, hoc est, vitra sou Crystallos pcllucidos, accidunt." 



THE DIRTH OF SPECTRUM ANALYSIS. 

incidit, obliquius igitur in E^i rcfringitur quam hie. Nam 
si minuas dea, crit et minuendus mkc, ex lege reper- 
cussus. Et sic deniquc EM in B C rectus incidct, itaque 




nihil in M refringetur. Cum autem Fii hoc pacto bis per- 
transicrit coqjus vitrt, quippe semel in de, itenimin em, 
exiens recta per M, radium vitri colore j'aculatur in K, rectius 
tamen e regione ipsius A. Nam docemur ex opticis, radios 
iucidos tingi in mediis coloratis." 



Now the refraction or bending of light takes place when 
the ray passes obliquely from one medium to another of dif- 
ferent density, as from atr into water, or from water into air. 
A simple experiment may be made by passing the beam of 
light from above into a glass vessel containing water. If 
the ray strikes the surface perpendicularly, it will be seen 
that no visible change takes place ; the ray simply proceeds 
directly into the water without altering its direction. If, 
however, the beam be allowed to fall on the surface of the 
water, say at an angle of about 45°, two things may be 
observed. In the first place a reflection will take place at 
the surface of the water, that is to say, the light will appear 
reflected at the surface, and it will be noticed incidently 
that the angle at which the reflected ray leaves the water 
is precisely equal to that at which tlie incident ray strikes 



Rtfratl{«n 
■wlun tkf 




r 



Iht rtfratt- 
iHg medium 
IS changed. 



SOLAR PHYSICS. 

the surface, thus proving the rule that " the angles of inci- 
dence and of reflection are equal :" the second thing to be 
noticed is that on entering the water the direction of the 
beam of light will not be the same as it was in the air. 
In Fig. id, the ray R I, striking the water at I, instead of 




proceeding to r' is deflected or refracted to S ; that is, ihc 
ray will be bent downwards, or, what is the same thing, 
towards a line I P. perpendicular to the surface, to a definite 
extent, depending on the angle of the incident ray. The 
experiment may be varied by allowing the light to fall on 
the surface at various angles, when it can be shown that 




the angle formed by the ray refracted in the water ii 
according to the angle of the incident ray, and that the 
angles formed are bound together by a regular law. A 



THE BIRTH OF SPECTRUM ASALYSIS. 130 



very instructive experiment is to place a coin at the bottom chai . i\. 
of a vessel, and then, standing so that the ocnn is just hidden 
by its edge, to gradually fill the ixssel with water ; the cc»n 
with the bottom of the vessel will ai^>ear to rise, and will 
become visible, as shown in Fig. 37. 

The amount of refraction varies with the medium Tiu rm v 
employed, and also with its tc m pciature. The effect of dif- 7^^ 
ferent media can be clearly seen by passing a ray of light ^^^*^ '^ 
into a vessel, containing a liquid such as bisulphide of rr/raa^ 
carbon, with a layer of water floating on the top. The ««»»• "- 
ray will be seen to be bent on entering the water, and to 
be still more bent on passing from the water into the 
layer of bisulphide of carbon. 

We have now to see what takes place when a ray of 
light enters a piece of glass. We will take first the case of 
glass with parallel sides. The ray on entering the glass at 
the upper surface is refracted downwards, as in the case of 
water, and travels through the glass until it reaches the 
under surface. Here we have precisely the reverse condi- 
tion holding, — that is, the ray of light passes from a dense 
medium to a rarer one ; the ray is refracted upwards or 
away from the perpendicular line, and thus will exactly 
neutralize the previous refraction, and the beam of light 
will come out in a direction parallel to its original path, 
though not quite in the same straight line: as shown in 
Fig. 33, the ray, instead of proceeding in the direction of 
s', proceeds in the direction of S. 

So much for the deviation or refraction resulting from the Ti^'<' '> 
action of a prism upon light, but there is another effect : ''^■f/ ' 
the light, which was white on entering the prism, is separated 
into several colours on leaving it : the candle as seen in the 
experiment in fact is not white, but is fringed with colours. 
If we again take our beam of light in the dark room, as in 
Fig. 38, and allow it to strike on a prism, so placed that 
its edges are horizontal, and also that the beam enter its 
obliquely by one of its surfaces, and then receive the image 
on a screen, we see a band of colours which reminds us 



The rtfrd, 
ttd beam i 

(BloMTld. 



SOLAR PHYSICS. 

strongly of the rainbow : the lowest colour, if the base of 
the prism be upwards, will be red, next above oranj;i;, 
passing by imperceptible gradations to yellow, and after- 
wards green, which then passes through the shades of 
greenish blue till it becomes a pure blue, then indigo, and 
finally ends with a violet colour. Tlie transition from one 




colour to another is not abrupt, but is made in an imper- 
ceptible manner, so that it can scarcely be said, for id- 
stance, where the yellow ends or the green begins. 

The cause of this band of colours, or spectrum, as it is 
called, was first discovered by Sir Isaac Newton ; and bcrv 
we see the birth of Spectrum Analysis. He took one of 
the colours thu.f produced — say red, as is sliown in the 



THE BtKTH OF SPBCnVM AXAt.TstS^ 



figure — and made it piss 

the image «id a second scnxa ; the 

ntfaei- longer, but the 

periment provts that this cnlcvr cf the sftxXxm^ 




and the same has been found of all the others. As Nen-ton tnt^ mf 
in his experiment operated with sunlight, the band of *^,^^^ 
colours was in this case called ihe solar sfntnim. The 
rainbow is nothing more nor less than a solar spectrum. 
caused by refraction in the rain-drops. 

Let us next take two beams of differently coloured light, 




difftrtHt 

roloHn an 



red and blue for instance, and pass them through the same 
prism. We see that the action of the prism on these two 
differently coloured beams is unequal ; in other words, we 
get the red beam deflected to a certain distance from a 
straight line, and the blue deflected to a certain other 
distance. We see by this experiment that there is a dis- 
tinct difference in the amount of refrangibility — that the 
red light is not diverted so far out of its original direction 
by the prism as the blue. And this leads us to Newton's 
first proposition, which is this: " Lights which differ in 
colour differ in refrangibility" • This requires no explana- 
tion ; it may be translated^ Lights which differ in colour 
arc differently acted upon by a prism. 




We now approach Newton's great discovery, which is 
this : " The light of the sun eonsists of rays differently re- 
frangible" 

Let us take a beam of sunlight this time, and make it 
pass through a prism, bearing in mind the former experi- 
ment : we shall sec then that the action of the prism is at 
once to turn that beam into a beautifully coloured band, 
which reminds us of a rainbow. It was this which Newton 
did in a dark room, which led him to his important dis- 
covery— that zt'hite light is compounded of light of different 
' Sec -Appendix f«r cKiracis from Newton's " Optic*.' 



THE BIRTH OF SPECTRUM ANALYSIS. 



IM 



dtgrees of refrangibiiily. But how is it possible to show chap. ik. 
the truth of Newton's assertion that white light is com- 
pounded of these ditTerent colours? We can do so by ligkiUn- 
simply placing in the path of the coloured beam another /""'•*'*'■ 
prism placed in a contrary direction, as shown in Fig. 40. 
In the same way white li^ht may be reproduced by means 
of a lens (Hig- 41)- We see in a moment that we get 




back white light ; for the second pnsm exactly neutralizes 
the effect caused by the first, and the ray proceeds as if 
nothing had happened, 

This can be shown by an experiment of a different order. 
If a disc, divided into sectors and coloured with the prin- 
cipal colours of the spectrum, as shown in Fig. 42, be 
taken, and if it be true that the idea of white light is 
simply an idea built up by the eye, because we have all 
these multitudes of light-waves perpetually pouring into it 
with a velocity that is very much greater than anything 
which can be translated into words, surely we should get 
something like this effect also if we were able, by rapidly 
rotating the disc, to obtain a more or less perfect substi- 



.SOLAR PllYSrCS. 

tute for white liglit. The coloured disc being made to 
rotate rapidly, we obtain something like an approximation 
to white light. To show that this is really an cflfect due 
to the flowing in of light from differently coloured parts of 
the disc into the eye, and so forming this compound 
impression which is conveyed to the brain, we can var)- 




the experiment in thiswise. Instead of illuminating the 
disc continuously by the electric lamp, or by sunlight. 
illuminate it intermittently, by an electric spark; then, 
although the disc is rotating rapidly all the time; each 
separate colour is discernible, and the disc appears to stand 
still. The reason of this difference is, that in one case the 
rotation of the wheel builds up a compound image in the 
eye, and in the other case it cannot do so, because the 
flash of the light is much more rapid and tnstantaneou.s 
than the rotation of the wheel. 

There is one more experiment which can be easily made, 
to show that all the beautiful colour which we get in 
natun: is really reflected after all. and that if our sunlight. 



THE BIRTH OF SPECTRUM ANALYSIS, 145 

instead of being polychromatic — that is to say, compounded chap. ix. 
of all these beautiful colours — were monochromatic, or of 
one colour only, the whole expanse of creation would put monochro- 
on a very different appearance from what it does. If, ^f^^^^^^s^^- 
instead of illuminating a diagram, the letters of which are 
of different bright colours, by the white light of the 
electric lamp, we illuminate it by a light that only contains 
one colour — by the yellow light of sodium, for instance — and 
then look at it, we see that some of the letters upon it are 
almost invisible, whilst others are very clear, the yellow 
light only allowing a difference to be seen of more or less 
depth of shade — there being no difference in colour. But 
if we allow the polychromatic light of an electric lamp or 
of the sun to fall upon the diagram, we at once see all 
the letters in different colours. This experiment feebly 
indicates the advantage we possess in living in a uni- 
verse lit by white, or polychromatic, or many-coloured 
light, instead of light which is merely blue, or yellow, or 
any other single colour. 

Hitherto we have spoken of refraction. We will now 
introduce the word dispersion^ which represents simply a Dispersion, 
measure of different refractions, or the difference between 
the bending of the red and the violet rays of light. In an 
ordinary spectrum, the difference between the red and the 
violet is the difference of the refractions of those two 
colours by the prism ; and the angle which the red, or 
yellow, or other colour, forms with the original path of the Angle of 
compound-beam, is called the angle of dcination of that «''^^'^''' 
colour. 

There is one other consideration which we owe to 
Newton. In his very first experiments, that great philo- 
sopher discovered that the quality of the spectrum de- 
i:>ended very much on the following consideration : — If we 
wish to get the best possible effect out of a prism, and the 
purest possible spectrum, we have so to arrange it that 
the particular ray which we wish to observe, whether the 
yellow, the blue, the green, or any other, leaves that prism 

L 



146 



SOLAR PHYSICS. 



CHAP. IX. at exactly the same angle as the incident compound ray 

~ Afi^e~of ^^^^^ ^^ *^- This angle is termed the angle of minimum 

tninirtum deviation, 

./. .//w//. j^ jg ^^^y curious, however, that Newton, although he 
made many experiments on prisms, really omitted one of 
the most important points ; and here again we get an 
idea of the enormous patience which is necessary in these 
matters, for we had to wait a century and a quarter before 
the next essential point was hit upon which has helped us 
in our study of the solar spectrum. Newton made a round 
or oblong hole in a shutter for his experiments, but we 
now know he ought not to, have done that ; he ought to 
have made a slit. But this did not come out until 1802, 

W'oiUuton when Dr. Wollaston, by merely using a slit instead of a 

tluliVhi ^^^"^^ hole, made a tremendous step in advance — the real 
1802. basis of all the modern work which has been done in 
solar physics by means of the spectro.scope. The im- 
portance of this is obvious.. Suppose we take a cylindrical 
beam of sunlight and put a prism in the path of the 
beam, we observe that the spectrum is not a pure one; 
but if we change the round hole for a slit, we obtain 
a spectrum of the greatest purity : the red, blue, green, and 
violet, instead of overlapping and destroying the beauty of 
the spectrum, show distinctly as simple colours, each one 
speaking for itself on the screen. By using this narrow 
slit instead of the round hole which Newton made in the 
shutter, we got the first idea of the tremendous importance 
of spectrum analysis ; for no sooner had Dr. Wollaston 
examined the sunlight with the new arrangement, as 
Newton had done a century and a quarter before with the 
old one, than he found out that it was not at all as Newton 
had represented it. Newton told us, in fact, that the sun- 
light was continuous ; that is to say, that the spectrum was 
one in which there was no break in the light which flowed 
out to every part of the spectrum, from the extreme red to 
the violet. When Dr. Wollaston tried the slit, he found, 
however, that the spectrum, instead of being an unbroken 



THE BIRTH OF SPECTRUM ANALYSIS, 



147 



rainbow band of light, was really broken by a succession of chap. ix. 
fine — beautifully fine — black lines. 

Here is an extract from Wollaston's communication to 
the Royal Society : ^ — 



« 



I cannot conclude these observations on dispersion without re- 
marking that the colours into which a beam of white light is separated 
by refraction appear to me to be neither seven, as they usually are 
seen in the rainbow, nor reducible by any means (that I can find) to 
three, as some persons have conceived ; but that, by employing a very 
narrow pencil of lights four primary divisions of the prismatic spec- 
trum may be seen with a degree of distinctness that I believe has not 
been described nor observed before. 

" If a beam of daylight be admitted into a dark room by a crevice 
^ of an inch broad, and received by the eye, at the distance of ten or 
twelve feet, through a prism of flint glass free from veins, held near 
the eye, the beam is seen to be separated into the four following 
colours only : red, yellowish-green, blue, and violet ; in the propor- 
tions represented in Fig. 43. The line a that bounds the red side 



IVollas- 

tons 
paper. 





Fig. 43. —The first obikervation of Fraunhofcr's lines. 

of the spectrum is somewhat confused, which seemed in part owing 
to want of power in the eye to converge red light. The line B, between 
red and green, in a certain position of the prism, is perfectly distinct ; 
so also are D and E, the two limits of violet ; but c, the limit of green 
and blue, is not so clearly marked as the rest : and there are also, on 
each side of this limit, other distinct dark lines, /and ;^, either of 
which in an imperfect experiment might be mistaken for the boundary 
of these colours. The position of the prism in which the colours arc 
most clearly divided is when the incident light makes about equal 
angles with two of its sides. I then found that the spaces a B, B c, 
C D, D E, occupied by them, were nearly as the numbers 16 23, 36 25, 
Since the proportions of these colours to each other have been sup- 
posed by Dr. Blair to. vary according to the medium by which they 
are produced, I have compared with this appearance the coloured 
images caused by prismatic vessels containing substances supposed 
by him to differ most in this respect, such as strong but colourless 



Philosophical Transactions, 1802, part i. p. 378. 



L 2 



148 



SOLAR PHYSICS, 



CHAP. IX. 

JVollas- 

toft's 
papa . 



Frauh- 

hoft'r's 

pnpn\ 



nitric acid, rectified oil of turpentine, very pale oil of sassafras, and 
Canada balsam, also nearly colourless. With each of these I have 
found the same arrangement of these four colours, and, in similar 
positions of the prisms, as nearly as I could judge, the same propor- 
tions of them. 

'* But when the inclination of any prism is altered so as to increase 
the dispersion of the colours, the proportions of them to each other 
are then also changed, so that the spaces A C - c D, instead of being, 
as before, 39 and 61, may be found altered as far as 42 and 48. 

*^ By candle-light a different set of appearances may be distinguished. 
When a very narrow line of the blue light at the lower part of the flame 
is examined alone, in the same manner, through a prism, the spectrum, 
instead of appearing a series of lights of different hues contiguous, 
may be seen divided into five images at a distance from each other. 
The first is broad red terminated by a bright line of yellow, the second 
and third are both green, the fourth and fifth are blue, the last of 
which appears to correspond with the division of blue and violet in 
the solar spectrum and the line D of Fig. 43. 

" When the object viewed is a blue line of electric light, I have 
found the spectrum to be also separated into several images, but the 
phenomena are somewhat different from the preceding. It is, however, 
needless to describe minutely appearances which vary according to the 
brilliancy of the light, and which I cannot undertake to explain.*' 

Although these lines were observed by Dr. Wollaston, it 
was not until 18 14 that we find them mapped out with the 
greatest care, to the number of 576, by a German optician 
named Fraunhofer, whose work was quite independent of 
WoUaston's ; hence they are termed " Fraunhofer lines," 
the principal ones being lettered A, B, C, &c. 

Fraunhofer's work will be gathered from the folIowiI^][ 
extract from his communication to the Munich Academy:* 

** Into a dark room, and through a vertical aperture in the window- 
shutter, about 15" broad and 36" high, I introduced the rays of the 
sun upon a prism of flint-glass placed upon the theodolite ; this instru- 
ment was 24 feet from the window, and the angle of the prism was 
nearly 60°. The prism was placed before the object-glass of the 
telescope so that the angles of incidence and emergence were equal 
In looking at this spectrum for the bright line which I had found 
in a spectrum of artificial light, I discovered, instead of this line,an iii- 
Jinite number of vertical lines of different thicknesses. These lines ait 
darker than the rest of the spectrum, and some of them appear 
entirely black. When the prism was turned so that the angle of 
incidence increased, these lines disappeared, and the same thing 



' Denkschriften tier K. Acad, der Wissenschaften zu AfitHckeu^ 
1814-15, Hand 5, pp. 193- 226. Translated in Edinburgh Philt^ 
sophical Journal, vol. ix. p. 296, and vol. x. p. 26, 1823. 



THE BIRTH OF SPECTRUM ANALYSIS, 



149 



happened when the angle was diminished. If the telescope was 
considerably shortened, these lines reappeared at a greater angle of 
incidence ; and at a smaller angle of incidence the eye-glass required 
to be pulled much farther out in order to perceive the lines. U the 
eye-glass had the position proper for seeing distinctly the lines in the 
red space, it was necessary to push it in to see the lines in the violet 
space. If the aperture by which the rays entered was enlarged, the 
finest lines w^ere not easily seen, and they disappeared entirely when 
it was about 40". 

" If it exceeded a minute, the largest lines could scarcely be seen. 
The distances of these lines and their relative proportions suffered no 
change, either by changing the aperture in the shutter, or varying the 
distance of the theodolite. The refracting medium of which the 
prism is made, and the size of its angle, did not prevent the lines 
from being always seen. They only became stronger or weaker, 
and were consequently more or less easily distinguished in proportion 
to the size of the spectrum. The proportion even of these lines to one 
another appeared to be the same for all refracting substances ; so that 
one line is found only in the blue, another only in the red, and hence 
it is easy to recognize those which we are observmg. The spectrum 
formed by the ordinary and extraordinary pencils of calcareous spar, 
exhibited the same hnes. The strongest lines do not bound the 
different colours of the spectrum, for the same colour is almost 
always found on both sides of a line, and the transition from one 
colour to another is scarcely sensible. 

" Fig. 44 shows the spectrum with the lines such as they are actually 
observed. It is, however, impossible to express on this scale all the 
lines and the modifications of their size. At the point A the red nearly 
terminates, and the violet at I. On either side we cannot define with 
certainty the limits of these colours, which, however, appear more 
distinctly in the red than in the violet. If the light of an illuminated 
cloud falls through the aperture on the prism, the spectrum appears 
to be bounded on one side between G and H, and on the other at B : 
the light of the sun, too, of great intensity, and reflected by aheliostatc, 
lengthens the spectrum almost one-half. In order, however, to observe 
this great elongation, the light between C and G must not reach the 
eye, because the impression of that which comes from ihe ex- 
tremities of the spectrum is so weak as to be extinguished by that 
of the middle of the spectrum. At a we observe distinctly a well- 
defined line. This, however, is not the boundary of the red, which 
still extends beyond it. At a there is a mass of lines forming together 
a band darker than the adjacent parts. The line at B is very distinct, 
and of a considerable thickness. From c to D may be reckoned nine 
very delicate and well-defined lines. The line at c is broad and black like 
I). Between C and D are found nearly thirty ver>' fine lines, which, 
however, with the exception of two, cannot be perceived but with a 
high magnified power and with prisms of great dispersion ; they are 
besides well-defined. The same is the case with the lines between B 
and c. The hne D consists of two strong lines separated by a bright 
one. Between i> and K we recognize about eighty-four lines of dif- 
ferent sizes ; that at E consists of several lines, of which the middle one 
is the strongest. From K to h there are nearly twenty-four lines ; at 



CHAI*. IX. 

Fraun- 

hofii's 
paper. 



SOLAR PHYSICS. 




THE BIRTH OF SPECTRUM ANALYSIS. 151 



b there are three very strong ones, two of which are separated by a chap. ix. 

tine and clear line ; they are among the strongest in the spcctrLm. 

The space b F contains nearly fifty-two lines, of which F is very strong. Frayn- 
Between F and G there are about 185 lines of different sizes ; at G hoftrs 
many lines are accumulated, several of which are remarkable for their P^t^- 
size. From G to H there are nearly 190 different lines. The two 
bands at H are of a very singular nature : they are both nearly 
equal, and are formed of several lines, in the middle of which there 
is one very strong and deep. From H to i they likewise occur in 
great numbers. Hence it follows that in the space B H there are 
574 lines, the strongest of which are shown in the figure. The 
relative distances of the strongest lines were measured with the 
theodolite, and placed in the figure from observation. The faintest 
lines only were inserted from estimation by the eye. 

"Various experiments and changes to which I have submitted these 
lines convince me that they have their origin in the nature of the 
light of the sun^ and that they cannot be attributed to illusion, to 
aberration, or any other secondary cause." 

He then points out that in transmitting the light of 
a lamp through the same aperture we observe a line in the 
yellow which occupies exactly the same place as D in the 
solar spectrum. We shall return to this point presently. 

The paper then goes on : — 

" It is easy to understand why the lines are not well marked, and 
why ihey disappear if the aperture of the window becomes too large. 
The largest lines occupy nearly a space of from 5" to 10". If the 
aperture is not such that the light which passes through it cannot be 
regarded as a single ray, or if the angle of the width of the aperture 
is greater than that of the width of the line, then the image of the 
same line will be projected several times parallel to itself, and will 
consequently become indistinct, and disappear when the aperture is 
too great. The reason why in turning the prism we cease to see the 
lines unless the telescope is lengthened or shortened, may be thus 
explained : — 

** The emersion of the rays in respect to their divergence is similar 
to their immersion only in the case where the angles of incidence and 
emergence are equal If the first angle is greater, the rays after re- 
fraction will diverge, as it were, from a more distant point ; and if it is 
smaller, from a nearer point. The reason of this is that the path of 
the rays which pass nearer the vertex of the prism is shorter than 
that of those which pass at a greater distance from the vertex. Hence 
the angles of the refracted rays are not changed, but the sides of the 
triangles for the emergent rays ought to be in the one case greater 
and in the other smaller. This difference oui^ht to vanish if the rays 
fall in parallel directions on the prism^ which is also proved by experi- 
ment. As the violet rays have by the object-glass of the telescope, 
though achromatic, a focal distance a little shorter than the red rays, 



152 



SOLAR PHYSICS. 



cHAi'. IX. wc see clearly why it is necessary to displace the eye-glass in order to 

— ; perceive the lines distinctly in the different colours. 

l^raun- «as the lines of the spectrum are extremely narrow, the apparatus 
^^'' ^ must be very perfect in order to avoid all aberration, by which the 
paper. \xxi^^ may be rendered indistinct and even dispersed. The sides of 
the prism ought consequently to be perfectly plain, and the glass of 
which the prisms are made ought to have neither scratches nor striae. 
With English flint-glass, which is never entirely free of these stria?, 
we can only see the strongest lines. Common glass, and even the 
English crown glass, contain many striae, though they are not always 
visible to the eye. Those who cannot procure a perfect prism of 
flint-glass should use a fluid of great dispersive power, such as oil of 
anise-seeds, in order to see all the lines. In this case the prismatic 
vessel ought to have its sides perfectly plane and parallel. In general 
the sides of all the prisms should form an angle of 90*^ with their 
base, and this base ought to be placed horizontally before the tele- 
scope if the axis of the telescope is horizontal. The narrow aperture 
by which the light passes ought to be exactly vertical. The reason 
why the lines become indistinct if any of the conditions now men- 
tioned are neglected, may now be readily understood." 

If, therefore, we say that solar spectroscopic inquiry 
dawned with Newton, certainly the sun began to rise with 
Wollaston and Fraunhofer. 

The solar spectrum, then, as we have said, far from being 
continuous, is crossed by an almost innumerable number of 
dark lines, some being fine and others thicker and blacker. 
But our knowledge of the spectrum is now very much 
more complete than in Fraunhofer's time, as our means of 
observing it have been enormously developed. 

We have now to pass on about thirty years, when Mr 

Theim- Simms,^ an optician of world-wide reputation, and Pro- 

i>rovement5 fessor Swan,^ independently of each other, made another 

of StmtHb . . * ^ • ^1 ^ W.J 

very important improvement in the spectroscope. Instead 
of merely using a prism and observing the slit with the 
naked eye, they placed a lens in front of the prism, s(» 
arranged that the slit was in the focus of the lens. 
The light which is allowed to pass through the slit is 

' Mem. R. A. S. 1839, vol. xi. pp. 168 and 169. Mr. Simms in this 
paper (describing the measurement of the refractive index of the optical 
glass prepared by the late Dr. Ritchie) states, " The only novelty of 
any consequence in this instrument [the spectroscope employed] i> 
the substitution of a collimator in place of a slit in a window shutter." 

2 Trans. Roy. Soc. Kdin. 1847 and 1856. 



and 
Swa<n. 



THE BIRTH OF SPECTRUM ANAL YSIS. 1 53 



thus turned into a cylindrical beam, and thus the light chap. ix. 
falls parallel on the full face of the prism, as Fraun- 
hofer suggested it might do (and which method he seems, 
from the part of his paper near the bottom of page 
151 I have italicized, to have used), with great advan- 
tage as to the quantity of light used and also to fine 
definition. Then, instead of having merely the eye to 
observe the spectrum, as Wollaston had, Fraunhofer's 
observing telescope is retained, which grasps the circular 
beam and compels it to throw an image of the slit, 
which may be magnified at pleasure. We thus see 
how closely connected are the grandest discoveries with 
the skill and suggestiveness of those who supply different 
instruments for our use. In the instrument then devised 
by Swan and Simms, we have the modern spectroscope. 
We have next to trace its growth, and the influence of 
its growth upon the study of solar physics. 



THE MODERN SPECTROSCOPE. 



^HAi\jc^ How then do we in our modern sun-work get beyond the 
Effect of *'^s^'^s achieved by Fraunhofer ? The reply to this ques- 
the density tion in the main is that we employ more dispersion^ but 
on fts]7is- ^^^^^ points have also been alluded to. Let us consider 
fcrsion. the question of dispersion first. As we have already seen, 
dispersion is the measure of the difference of the refrangi- 
bilities of the variously coloured rays. If we take a prism 
which appears like an ordinary one, but really is composed 
of several layers of different kinds of glass, arranged hori- 
zontally, and pass a beam of light through it, the beam 
will be differently acted upon by the various layers, and 
we shall get a difference in the dispersion. We shall have 
several distinct spectra, showing that there is something 
in the different layers of which this prism is composed 
which turns the light out of its path, and which disperses 
it more in some cases than it does in others. The cause 
of this is the different density of the glass composing 
each layer : some kinds of glass are nearly twice as 
heavy as others. It is a very natural conclusion, there- 
fore, that the heavier and denser glass should have a 
stronger action on the light, should give a wider spectrum, 
and thus enable us to study it better, than the lighter 
glass does — and fortunately we are not limited to glass ; 
for if we were, we should not be able to go so far in these 
in(|uiries as we do. So that, if we want great dispersion. 



THE MODERN SPECTROSCOPE. ISS 



we must use heavy glass, or leave glass behind alto- chap. x. 
gether, as amongst the liquids we find some, such as bi- 
sulphide of carbon, which give even a greater dispersion 
than the densest glass. There is another consideration. 
The dispersive power and refractive power depend not 
only upon the density of the glass, but on the refracting 
angle of the prism. If a beam of light is sent through Effect of 
two prisms of unequal angles, the effect is extremely ^in^a^^g 
distinct. Thus, if we take one prism with an angle of 20®, of the 
and another with an angle of 60°, the larger angle gives -^'^^'• 
a much greater deviation and dispersion. And now a 
third important point. There is no reason why we 
should not employ many prisms, and in practice this is 
done. First, then, we have a single prism of a dense sub- 
stance ; by increasing the angle we get increased dis- 
persion, and then we get it still further increased by adding 
another prism, and so we go on, adding prism after 
prism, until we get to a large number of prisms arranged 
in the best possible manner for the light to be successively 
dispersed by each of them, until at last we get a dispersion 
of such an enormous amount, that the spectrum of the 
sun, as mapped by Kirchhoff and Bunsen, is several yards 
in length, although it is nothing but a succession of images 
of one of the finest slits which our best opticians are able 
to make. 

We have before seen that our knowledge of the solar spec- 
trum depends first of all on Newton's work with the prism in 
1675, and on the fact which Newton found out incidentally, 
that it is important that the prism should be used at the 
angle of minimum deviation. We then get the slit added 
by Wollaston in 181 2 ; then the collimating lens added 
by Swan and Simms, about 1840; and now we have 
arrived at the spectroscope improved and modified as an 
instrument, until at last we get spectroscopes so arranged 
that the glass is of the finest possible materialy and the densest 
possible^ the angle the largest possible, and the number of 
prisms as great as possible. 



SOLAR PHYSICS. 




THE MODERN SPECTROSCOPE. 



prism instrument. When, however, any accurate and 
elaborate work has to be done, such as in carrying out 




many investigations, more prisms have to be employed. 
The engraving given in Fig. 47 represents an instrument 




which historically is extremely interesting, as being the 
one with which Kirchhoff made his most elaborate and 




SOLAR PHYSICS. 



k« p.,. 







THE MODERN SPECTROSCOPE. 

inquiries. Spectroscopes of many prisms can only be em- cha 
ployed in the case of strong lights, such as that of the 
sun or the electric arc, as the light is much dispersed or 
spread out, and much is lost by reflection. 

As the principle of construction is almost the same in all Dan 
kinds of spectroscopes, it will be well to commence with ^," 
a description of the simplest form, namely that with one 



'59 I 




prism, as .shown in Fig. 49. It will be seen to consist of a 
circular table, supported by a pillar and three legs, carrying 
three lateral tubes: the right-hand tube is called the colli- 
mator, and holds at its outer extremity the fine slit, the 
width of which can be regulated to a nicety by a micro- 
meter screw; the other end of the collimator is furnished 



i6o SOLAR PHYSICS, 



CHAP. X. with a lens, which serves to collect the rays of light coining 
from the slit, and to render them parallel before falling on 
the prism in the centre of the table. The prism is so 
placed and fixed by a clamp that the light entering the 
slit from the source of light, shown in the figure as a gas 
lamp, strikes it and leaves it at what is called the angle 
of minimum deviation, a term which has already been 
explained ; after passing through the prism, in which the 
light undergoes both deviation and dispersion, the spoctnim 
is observed by the telescope on the left, which is simply a 
small astronomical telescope of low magnifying power. 
Methods of There are two methods of measuring the position of the 
i^TposUhns '*"^'*^ *^ spectra. The telescope may be attached to a 
of the lines, movable arm, which can be directed to any part of the 
spectrum that may be required ; and the outer edge of the 
circle along which the telescope moves, may be graduated 
with an accurate scale of degrees, which can be divided 
with more or less minuteness, according to the precision in 
the exact position of the dark lines, &c. in various spectra 
required. In this method the line to be measured is 
brought into the centre of the field of view of the obser\*- 
ing telescope, and the position of the telescope read off. 
Of course if the line measured is situated in the red end 
of the spectrum, the telescope will be in a different position 
from that it would occupy if the line were in the blue end. 
The second method of measurement may be gathered 
from Fig. 49. It consists of a short tube carrying at its 
outer extremity a small photographic scale, which is illumi- 
nated by a candle flame ; the beam passing from the pho- 
tographic scale is rendered parallel and thrown on the 
surface of the prism by means of a lens in the tube carr>'- 
ing the scale, and is reflected by the last surface of the 
prism up the observing telescope, so that it is seen as a 
bright scale on the background formed by the spectrum 
under observation. 

By covering up one part of the slit by a reflecting 
prism, it is possible to observe the spectra of two light- 



THE MODERN SPECTROSCOPE. 



sources at the same time. The modus operandi will be chap. 
gathered from the accompanying woodcuts. 




We have now an idea of the action of the simple prism. 
Another kind of prism, which differs from the simple one Dtrtti- 
very much as the achromatic telescope differs from the non- '*™" 




achromatic one — which was the first attempt made at an 
instrument for astronomical observation — is next to be 
noticed. The object-glass of a telescope.asnowconstructed, 
consists of two lenses made of different kinds of glass. Of 
course, we have dispersion and deviation at work in both 



SOLAR PHYSICS. 



these kinds of glass, but the lenses are so arranged, and 
their curves are so chosen, that, as a total result, the 
deviation is kept while the dispersion is eliminated, so that, 
in the telescope, we have a nearly white image of anythinjj 
which gives us ordinary light, although it is by the devia- 
tion alone that we are enabled to get the magnified image 
of that object. So also in the spectroscope we have an 
opportunity of varying the deviation and the dispersion. 
By a converse arrangement we can keep the dispersion 
while we lose the deviation ; in other words, we have what 
is called a direct-vision spectroscope. If we take one com- 
posed of two prisms of one kind of glass which possesses 
a considerable dispersive power, and three prisms of another 




kind which does not disperse so strongly, arranged with 
Digtrrni their bases the opposite way, the deviation caused by the 
eh^i'af '*° prisms in the one direction will be neutralized by the 
<lifereni deviation of the three prisms in the opposite direction . 
'''^y- whilst the di^pcrsir.ii l,y the t«o prisms c>^cceds that wh.cli 




k 



is caused by the three prisms in the opposite direction ; 
the latter dispersion, therefore, will neutralize a portion 
only of the dispersion due to the two prisms. The final 
result is that there is an outstanding dispersion i 



sion after the J 



THE MODERN SPECTROSCOPE. 



'63 



deviation has been neutralized, so that when we want to chap. ) 
examine the spectrum of an object we no longer have to 
look at it at an angle. We have an opportunity, by this 
arrangement, of seeing the spectrum of an object hy looking 
straicjht at the source of light : in the application of spec- 
trum analysis, especially to the microscope and telescope. 
this modification — due to M. Janssen, the well-known 
astronomer, who was the first to bring it into general notice 
— is one of great practical importance, so that in any re- 
search which does not require excessive dispersion, this 
direct-vision arrangement is getting into common use. 

We must now go a tittle more into detail, principally DuaUm 
with regard to the spectroscopes used in connection with *""''/'"" 




telescopes for work on the so!ar and other spectra. One 
class of spectroscopes thus applied to telescopes is arranged 




J 



SOLAR PHYSICS. 



\f. X. for observing the spectra of the sun wketi great dispentm 




is not riqiiircd : these, therefore, serve at the same time 
for observation of the spectra of stars, nebulae, &c, and 




anotlier witli a much greater dispersive power for observing 



THE MODERN SPECTROSCOPE. 16; 



the spectrum of the sun specially. In both spectroscopes the chap. x. 
arrangements employed are similar, and include those of the 
instruments that have been already described, but there are ^ 

others necessitated by the attachment to the spectroscope. /^ uUnopl. 
The spectroscope is attached to the eye-piece end of the 
instrument, and the image formed by the telescope is re- 
ceived on the slit plate. A finder on the side of the 
telescope is used which enables the image of the star to 
be brought on the slit of the spectroscope ; while, in the 
case of the sun, its image can be thrown on the slit 
screen without such aid, and any part of the image may 
be brought on the slit by mere inspection. Arrangements 
are necessary in the case of the star spectroscope (i) for Necessities 
widening out the spectrum — this is done by a cylindrical s/>ectro' 
lens ; and (2) for obtaining a spectrum of comparison — ^'^'A- 
this is done by reflecting into the instrument the light 
of the vapours to be compared, rendered incandescent by 
an electric spark. Spectroscopes of this class are also 
made with direct-vision prisms, as in the accompanying 
one used by Father Secchi. 




Fig. 58. — Direct-vision star spectroscope. (Secchi.; 

In the spectroscopes in which great dispersion is not 
required, the number of prisms, and the consequent devia- 
tion and dispersion, are small. The accompanying wood- 
cuts will make their detailed construction quite clear. In 
the case of spectroscopes used only for the sun, the de- 
viation and dispersion required are large, the deviation 
amounting to over 300" ; that is to say, the ray of light is 



SOLAR PHVSrCS. 

bent through almost a complete circle ; the light fiom stars 
is dim, and many prisms cannot be employed to widen out 
the spectrum, but, in the case of the sun, there is light 





Figs. 59, 60, and 61 show very powerful spectroscopes 
to be attached to the telescope for observing the spectrum of 
the sun. One peculiarity of the instrument in Fig. 61 is tbiA 




the ray of light, having passed once through the lower part 
of the train of prisma, is received by a right-angled prism,* 
which totally reflects the light twice, sending the ray of light 




back through the upper part of the same prisms, when it is 
again refracted ; we thus have, by using these prisms, the 
same efTect as if thirteen prisms had been employed. The 
ray of light enters the instrument by the lower tube, and 
after passing first through the lower half of the prisms, and 
back through the upper half, is received in the upper tube, 
and reflected upwards for convenience of observation. These 

' 1 had a «pcclro$cop>e vrith a prism of this kind constniclcd in 
1869, and afterwards the same idea occurred independently to Pro- 
fessor Voung, as also that of mounting the prisms on a spring, which 
had been suggested to me by my friend Mr. C. W. Hemming, and 
introduced into the spectroscope 10 which I have referred. 




SOZ^/f PHYSICS. 

prisms are so arranged, that whatever part of the spectrum 
is being observed, they are always at the angle of minimum 
deviation for this part of the spectrum — a very important 




point, as we have seen. This is done cither by attaching 
the prisms to a spring of ebonite or ^n-metal moving 
on a fixed point near the first prism of the scries, as in 
the arrangement shown in Fig. 6i, or each prism may be 
attached to a radial bar acting on a central pin, as shown 
in Fig. 62. 

Other extensions of the use of the spectroscope in the 
new method will be alluded to in the sequel. 



RADIATION AND ABSORPTION. 



Now that the construction and use of the spectroscope ( hap. xi. 
have been fully gone into, we come to the principles on 
which the science of spectrum analysis depends. We first 
saw Newton founding this new science, by using a hole in 
a shutter, admitting a beam of sunlight, and analysing it by 
means of the prism ; then we discussed Wollaston's and 
Fraunhofer's substitution of the slit, and instantly we heard 
of dark lines in the solar spectrum, and bright lines when 
other light-sources were examined. The foundation of 
solar chemistry has resulted from the explanation and cor- 
relation of these dark and bright lines. To get at the 
principles we must first deal with the different modes in 
which light is given out or radiated by various bodies 
under different physical conditions — ^with, in fact, the ra- 
diation of light. 

Now, if we take a platinum wire and heat it to red- Effect ot 
ness, and examine by means of the spectroscope the light ^^^'^ "" " 
emitted, we shall find that only red rays are visible ; then idre. 
if the wire be gradually heated more strongly, the yellow, 
green, and blue rays will become visible, until finally, 
when the wire has attained a brilliant white heat, the whole 
of the colours of the spectrum will be present. If I burn 
a piece of paper, or a match, or ordinary coal-gas, I get 
a white light: in fact, if I raise any solid or liquid to a 



SADIATION AND ABSOliPTIOS. 

points, which are intensely heated by their resistance to • 
the passage of the current. The spectrum obtained from 
these points, by means of the dispersion of two bisulphide 
of carbon prisms, is quite continuous from end to end ; 




that is to say, there are no breaks, such as those Fraun- 
hofcr saw (Fig. 44), where the black lines represent the 
breaks in the solar spectrum which arc called the Fraun- 
hofer lines. 



SOLAR PHVSICS. 



1 



Let us then consider this fact established ; 
solid or liquid bodies, when luatcd to a I'ivii' 
give a coHtinnoHS spectrum wilhout bright 
these circumstances the light to the eye, wit 
troscope, will be white, like that of a white- 
the degree of incandescence is not so high, t 
only be red, like that of a red-hot poker, 
the spectrum goes^and it will expand towai 
as the incandescence increases, as before stal 
continuous. This is fact number one. 

Now, instead of observing the spectrum i 
white-light-giving carbon points or the li 
ordinary gas flame, let us examine the spectn 
source which is coloured. If, for instance, 
coloured fire, such as the red fire of our py; 
plays, on examining .such a light by meana 
troscope, we might expect that in the case 
should obtain the red end of the continuous s 
on burning green fire we should see the %n 
the spectrum, and so on. But this is not sof' 
the background of the spectrum is dark or 
that tve have certain localizatiuus of light , 
('// differeut parts of the spectrum. Now, i 
in colour are accompanied by dtfTercnces li 
We have something very different from tl 
spectrum we had before ; we have establishi 
between a solid or liquid body, which gives usi 
spectrum, and a vapour or gas which colout^ 
the spectrum of such a flame contains bright 

In these instances, then, the spectra coi 
which arc located in different parts of the sp 
us next burn some sodium in air, and then 
' spectrum of its vapour; or, better still, let u 
sodium, or a salt of this metal, such as table 
flame which is consuming a mixture of air 
burner known under the name of a Bunsen' 
bluish flame of which is due lo tli 



: completta 




Discontinuous 
AnsoRPiioN 
Spkctra, 



RADIATION AND ABSORPTION. 



173 



due to the greater supply of air from the holes at the chap, t 
bottom. The flame immediately becomes of an intense Buhuh 
yellow colour, due to the vapour of sodium. In this we have '*'■''"* 




further evidence of the connection between the colour of 
the light which we get from a vapour and the spectrum of 
that vapour. It is usual to place the salt to be examined in a 
platinum spoon, and insert it in the flame ; but the utmost 
constancy is insured by adopting an arrangement of Mit- 
scherlich's, shown in the accompanying drawing (Fig. 68), 
in which a platinum wick is kept continually moistened by 
a solution of the salt, generally the chloride, the spectrum 
of which is required to be examined. It will be imagined a 
priori, from what has been already said, that, as in the case 
of sodium vapour, the colour of the flame is orange, the 
line of the vapour will appear in the yellow or orange part 
of the spectrum, and we shall not be mistaken. It is seen, 
in fact, on examining this flame with a spectroscope, that 
the spectrum consists of a brilliant yellow line upon an 



Sfalrutii 

of !i-dium 



174 SOLAR PHYSICS. 

p. xr. almost black background ; if, howe%'er, the flame isobsend 
~~ by means of a very narrow siit, this line will appear double; 

it really consists of two extremely fine lines which are vot 

close to each other. 
ium Let us pass on to another substance, and take sonif; 
""'■■ lithium instead of sodium. A brilliant carmine-tinted 

flame is seen, which on examination by the spectroscope is 




Vk: fc*. -- Knii'.-i 

supply nf lioiihl 
plJlimim.icV 



found to give a spectrum consisting of one splendid red. 
and a fainter orange line. Potassium gives a violet-coloured 
flame, and yields in the spectroscope a red line and a violet 
line. If, again, we take a salt of strontium, one of the 
ingredients in red fire, it colours the fiame crimson, and by 
the eye the flame can scarcely be distinguished from the 
colour of the lithium flame, but in the spectroscope there 
is no possibility of doubt : the spectrum of strontium con- 
sists of a group of several lines in the red and orange, and 
a fine line in the blue end of the spectrum. 



RADtATION AND ABSORPTION. 



ir a higher temperature than that of the Bunsen flame is ' 
required, the blow-pipe flame {Kig. 69) may be resorted to ; 




in this the quantity of air and coal-gas or hydrogen is varied 
at pleasure, and a very high temperature may be obtained. 




Wc might proceed thus to examine the flame spectra of 
various salts ; but to observe the spectra of the vapours 



^ 



SOLAR PHYSICS. 

of the metals, it will be found necessary to use a higher 
temperature still, and for this purpose the electric arc or 
spark is employed. If a temperature only slightly greater 
than that of the blowpipe flame is used, the spark from 
an induction coil worked by five Grove cells may be talcen 
as shown in Fig. 71, the Leyden jar not being employed: 
a few metallic lines wilt then be seen, and a background 
consisting generally of bands of light here and there. 




If a higher temperature still is required, the jar must be 
thrown into the circuit, upon which the spark will become 
more intense, according to the power of the coil and 
size of the jar ; or the electric arc may be employed. 



w 



RADIATION AND ABSORPTION. 



le spectra thus obtained are much more complex ; the ' 
jctrum of iron, for instance, when examined at this high 
nperature, is found to consist of no less than 460 bright 
es, many of which are situated in the green part of the 
Eclrum. 




The manner in which the spectra of vapours may be Sf/tira r/ 
lenvisc obtained will be gathered from Fig. 72. There ^"''^ 
I also a great many gases which the spectroscopist 
V to study, and to study with great care. How is 
b managed ? Tubes containing gases are prepared ; and 
len we wish to study the spectrum of a gas, we do it 





in this way : we enclose it in a tube, and send a current 
through it by means of an induction-coil. If we pass a 
stream of electric sparks through a tube containing 
hydrogen, we shall see that the colour of 
the incandescent gas is a bright carmine- 
red, the spectrum of which can easily be 
observed by placing the spark tube 
front of the slit of one of the spccti 
scopes before clescrlbL-d. This arrange 
ment is one that is in daily use in man 
of our laboratories, and it must be bont 
in mind as being the moiiits ofieraadi b 
which a great deal of the work has 
dune to which I shall have to allmj 
shortly. 

To sum up, then, in all cases where IP 
are dealing with vapours and not tal 
or liquids, instead of a continuousspO 
trum we get a discontinuous one — Lt. % 
get bright lines. This is fact number tW 
In the two facts to which I have r 
fcrrcd we have, brought shaqily to 
focus, the labours of a long series 
illustrious investigators, from WoU 
whom, on page 148, we sau 
the spectrum of the electric light tO 
^ieparated into several images, 
lie " could not undertake to ex[ 
tiirough Hcrschel. Fox Talbot, WluJ 
stone. Masson. Angstrom, I'iuckcr, W. 
Miller. Swan, Kirchhoff, and Bunsen. 
'wge* '"■'"' '" Spectrum analysis, then, applied to the' 
radiation of light, teaches us that soltds 
ami liquids gh-e out cotiliuuoiis spectra, and that vapoHri 
and gases give out discontimtous spectra; that is to say, 
that we get bright lines in different parts of the spectrum, 
instead of having an unbroken light all over the spectrum. 



J 



1 


A'AD/A riON AND ABSOKl'T/UN. 179 


This statement may be varied by stating broadly that the cii*i-. xi 
radiation or giving out of light by solids and liquids is a ^^ 
general one, and that the radiation or giving out of light ^^^H 
by gases and vapours, instead of beint; general, is in the ^^^H 




1 


^_^^g 






^■^Bl 








^■^^1 






^^^^B 








^^^^s 








^^^^s 






^H^^S 








^^B^^Bi 








I^R^^I 




H 










I^B 










^^^^1 






III. -, -Wfc«w..n<.mj|:.-I >hi'|Til""l 'ht .JVoi..>..f si.n.^.if ihccJ-n.-,,,. . 

Further : t/u- s/vitruni given out by eafit element in a a 
state of gas is so difftrmt from the spectra of aJl other ' 
fUmenls. t/uit in eae/l case Ihr element to ivliicli the sftetrum d 
belongs can easily be detcnnitud. 

X 2 





SOLAR PHYSICS. 

We must now pass from the radiation or giving out ol 
light by bodies in different states— that is to say, by solid 
or liquid bodies, or gaseous or vaporous ones — to the 
action of the prism upon light under some new conditions. 
Light is not only given out, or radiated, but it may be 
stopf-ed or absorbed in its passage from the light-source to 
our eye, if we interpose in the path of the beam certain 
more or less perfectly transparent substances, be they solids 
liquids, gases, or vapours. I will recall one or two of the 
experiments to which reference was made, in order that 
it may be exactly seen how the perfectly distinct classes of 
phenomena due to radiation and absorption really nm 
together. It was pointed out that radiation, or the givii^ 
out of light, was coHtinuous or selective. 

Now, radiation is exactly equalled by absorption in this 
f matter; absorption may also be coittinuotis or selectivt- 
We took before, as an instance of continuous radiatioa, 
a continuous spectrum obtained by using the electric lamp 
or a hme-light : that is to say, an example of the gencnl 
radiation which we get from an incandescent solid — the 
carbon points of which the poles of the lamp are composed, 




or the solid lime. It will also be recollected that whcn« 
observed the spectrum of a vapour — as, for instance, tlu' 
iif strontium or thallium — that the continuous spcetru"' 



RADIATION AND ABSORPTION. i3r 

was altogether changed, and, in place of a beautiful rainbow chap. \ 
band, continuous from the red end of the spectrum to the 
violet, we really only got lines here and there, which are 
due to selective radiation, and opposed to the general 
radiation spoken of in the continuous spectrum just now. 
What I have to dwell on now is, that the absorption or 
sifting of light by different bodies is very like radiation 
in its results — that is to say, in some cases we have an 
absorption which deals equally with every part of the 
spectrum, and in other cases we have absorption which only 
picks out particular parts of the spectrum here and there 
to act upon. But there is one important point to 
be borne in mind : when dealing with absorption, we 
must always have a continuous spectrum to act upon. 
If we had a discontinuous spectrum to act upon, the 
thing would not be at all so clear. Having this con- 
tinuous spectrum, thr problem is, what the action of the 
different sub-stances on the light will be. Let me give you 
an instance of general absorption. If we take the con- 
tinuous spectrum above referred to, and interpose a piece 
of smoked glass, or better, a piece of neutral-tint glass, 
you will find that the substance will cut off tlie light and 




u udcs fur ^udyuig Lh 



deaden the spectrum, so to speak, throughout its whole 
length. This neutral -tinted glass, then, has the faculty /"■/.'"■ 
evidently of keeping back the light, red, yellow, blue, green, 
violet, and so on ; and is an instance of general absorption. 



SOLAR PHYSICS. 

A very dense vapour would furnish us with another similar 
instance. Now if, instead of using neutral-tint glass, a 
piece of coloured rIjiss is introduced, the action of thi^, 
instead of being general throughout the spectrum, will be 
limited to a particular part of it. Thus, a piece of red 
glass will cut off nearly all the light except the red: and 
a piece of blue glass will cut off everything except the 
blue. By introducing both these pieces in the beam, the 
spectrum is entirely obliterated. 

In these latter cases we have instances, not of general, 
but of selective .ibsorption, one substance cutting off every- 
thing but the red, and the other cutting off everything but 
■ the violet. Now, as different substances are kno«-n by 
their radiation, so also chemists find it perfectly easy to 
detect different substances by means of their absorption: 
for instance, the absorption spectrum of nitrous fumes can 




be shown by taking first our continuous spectnim, which 
we must always have to start with, and introducing some 
nitric peroxide between the source of light and the prism. 
Tlie nitric oxide, immediately it comes in contact with the 
air, produces drnsc red fumes, and numbers of fine black 
lines will he seen immediately crossing the s|>cclrum at 
right angles to its length, causing it to resemble the 



solar spectrum with its Fraunhofer lines. Iodine is < 
another substance which (fives a coloured vapour, the 
absorption spectrum of which is very definite and well 
defined. Fig. yy. Spectrum No. I, shows tlie absorption 
spectrum of iodine vapour, and No. 2 that of nitrt.us fumes. 



m 




i 



We are not hmited to these substances ; we may try blood, 
i^lbr instance. We shal! find that the action of blood upon 
[-the light is perfectly distinct from the action of those fumes 
Vhich we have spoken of; and instead of having typical 




lines in the green and blue parts of the spectrum, we have 
two very obvious bands in the more luminous part of the 
spectrum. The colour of a solution of blood is not unlike 




1 84 SOLAR PHYSICS. 



CHAP. XI. the colour of a solution of magenta ; but if, instead of 
using a solution of blood, we use a solution of magenta, 
we should have only a single black bands. The absorption 
spectrum of potassic permanganate solution is another 
beautiful instance. Instead of the two dark band which we 
saw in the case of the blood, or the single one in the case 
of magenta, we have four very definite absorption bands 
in the green part of the spectrum. So that the means of 
research spectrum analysis affords as far as regards radia- 
tion, is entirely reproduced in the case of absorption ; and 
it is perfectly easy, by means of the absorption of different 
vapours and different substances held in solution, to deter- 
mine not only what the absorbers really are, but to deter- 
mine the presence of an extremely small quantity. Further, 
by allowing the light to pass through a greater thickness 
of the absorbing substance, the absorption lines arc 
thickened and new regions of absorption are obsen'ed. 
This fact was discovered by Dr. Ghidstonc, who used 
hollow prisms containing the substance. 



HISTORY OF THE APPLICATIOX OF THE 
PRINCIPLES OF SPECTRLM ANALYSIS 
TO THE SOLAR SPECTRUM 

1 HAVE now to attempt t^ connect the two perfectly chap. xii. 

distinct classes of phenomena, to which I have referred in 

the last chapter — the phenomena, namely, of radiation and 

the phenomena of absorption ; and this connection between 

radiation and absorption is an instance of the slow growth 

of science. Fraunhofer, at the be^^^innin^ of this centurw 

had a ver^' shrewd suspicion of the p>erfect coincidence of 

place in the spectrum between certain dark lines which he 

saw in the spectrum of the sun. as we have seen p. 149^; 

and the bright lines in the spectrum of a lamp. Fraun- Fraunh.^- 

hofer at the first suspected, and after him many of our ^*^' ''^'•*- 

greatest minds suspected, that there was some hidden, 

wondrouslv stran;je. connection between the double vellow 

line which you will remember is characteristic of sodium, 

and a certain double line which exists among the 

black lines of the solar spectrum. Brewster many years 

worked on the same subject- I have been favoured by 

Dr. Gladstone with an e.xtract from Dr. Brewster's notfj- brra/u^r, 

book, dated St. Andrews, October 28th, 1841. In it '^^' 

Brewster says : — " I have this evening discovered the 

remarkable fact that, in the combustion of nitre up<>n 

charcoal, there are definite bright rays corresponding to 

the double lines of A and B, and the group of lines a 

in the space A B. The coincidence of two yellow rays 

with the two deficient ones at D, with the existence of 



1 86 



SOLAR PHYSICS. 



Fflucault^ 
1849. 



rHAP. xir. definite bright rays in the nitre flame, not only at D but 
at A, a and B, is so extraordinary that it indicates some 
regular connection between the two classes of phenomena.** 
Let me explain this at once in the light of modem 
science. 

The bright yellow line observed by Fraunhofer and 
Brewster is the bright line of sodium vapour, and we 
have established the third fact in spectrum analysis, that 
the vapours, the radiation of which has been referred to in 
Chapter XT., absorb, when relatively cool, the same rays 
which they emit . when hot. Hence, the absorption of 
sodium when cool gives us a dark line in the yellow, as 
its radiation when hot gives us a bright line in the yellow. 

A discovery made by Foucault in 1849 was the first to 
suggest this. In describing the result of the pri.smatic 
analysis of the Voltaic arc formed between charcoal poles, 
he remarked : ^ — 

"The spectrum is marked, as is known, in its whole 
extent by a multitude of irregularly grouped luminous 
lines ; but among these may be remarked a double line 
situated at the boundary of the yellow and orange. As 
this double line recalled, by its form and situation, the line 
D of the solar spectrum, I wished to try if it corresponded 
to it, and, in default of instruments for measuring the 
angles, I had recourse to a particular process. 

** I caused an image of the sun, formed by a converging 
lens, to fall on the arc itself, which allowed me to observe 
at the same time the electric and the solar spectrum super- 
posed ; I convinced myself in this way that the double 
bright line of the arc coincides exactly with the double 
dark line of the solar spectrum. 

" This process of investigation furnished me matter for 
some unexpected observations. It proved to me, in the 
first instance, the extreme transparency of the arc, which 
occasions only a faint shadow in the solar light. It showed 

* I] Institute Feb. 7. 1849, fanslated by Prof. Stokes, in PkiiM^if. 
vol. xix. p. 194. 



HISTORICAL NOTICE, 187 



me that this arc, placed in the path of a beam of solar chap, xii. 
light, absorbs the rays D, so that the above-mentioned line 
D of the solar light is considerably strengthened when the 
two spectra are exactly superposed. When, on the con- 
trary, they jut out one beyond the other, the line D 
appears darker tlian usual in the solar light, and stands out 
bright in the electric spectrum, which allows one easily to 
judge of their perfect coincidence. Thus the arc presents 
us with a medium which emits the rays D on its own 
account, and which at the same time absorbs them when 
they come from another quarter. 

"To make the experiment in a manner still more 
decisive, I projected on the arc the reflected image of one 
of the charcoal points, which, like all solid bodies in ignition, 
gives no lines ; and under the.se circumstances the line D 
appeared to me as in the solar spectrum." 

The explanation of this coincidence between the two stoka, 
bright lines of burning sodium vapour, and the two dark ^ ' 
lines D in the solar spectrum, which extended the grasp of 
spectrum analysis from terrestrial substances to the skies, 
w;is first given by Professor Stokes about 1852. 

The observational and experimental foundation on which 
Stokes based his explanation, has thus been stated by 
Sir William Thomson : ' — 

" I. The discovery by Fraunhofer of a coincidence Thefnts 
between his double dark line D of the solar spectrum and *y*'^^^"='*- 
a double bright line which he observed in the spectra of 
ordinary artificial flames. 

" 2. A very rigorous experimental test of this coincidence 
by Prof. W. H. Miller, which showed it to be accurate to an 
astonishing degree of minuteness. 

" 3. The fact that the yellow light given out when salt 
is thrown into burning spirits consists almost solely of the 
two nearly identical qualities which constitute that double 
bright line. 

' President's Address. British Association Meeting. 1871. 



1 88 SOLAR PHYSICS. 

cwAi». XII. '*4. Observations made by Stokes himself, which showed 
the bright line D to be absent in a candle-flame when the 
wick was snuffed clean, so as not to project into the 
luminous envelope, and from an alcohol flame when the 
spirit was burned in a watch-glass. And 

" 5. Foucault's admirable discovery, already referred to, 
that the Voltaic arc between charcoal points is ' a medium 
which emits the rays D on its own account, and at the 
same time absorbs them when they come from another 
quarter.' 



> »» 



//is con- The conclusions, theoretical and practical, which Stokes 
c ustons. tj^yghi; ^Q gi,. William Thomson, and which the latter gave 

regularly afterwards in his public lectures in the University 
of Glasgow, were : 

"I. That the double line D, whether bright or dark, is 
due to vapour of sodium. 

*' 2. That the ultimate atom of sodium is susceptible of 
r^egular elastic vibrations, like those of a tuning-fork or of 
stringed musical instruments ; that like an instrument with 
two strings tuned to approximate unison, or an approxi- 
mately circular elastic disc, it has two fundamental notes 
or vibrations of approximately equal pitch ; and that the 
periods of these vibrations are precisely the periods of the 
two slightly different yellow lights constituting the double 
bright line D. 

** 3. That when vapour of sodium is at a high enough 
temperature to become itself a source of light, each atom 
executes these two fundamental vibrations simultaneously; 
and that therefore the light proceeding from it is of 
the two qualities constituting the double bright line D. 

•* 4. That when vapour of sodium is present in space 
across which light from another source is propagated, its 
atoms, according to a well-known general principle of 
dynamics, are set to vibrate in either or both of those 
fundamental modes, if some of the incident light is of one 
or other of their periods, or some of one and some of the 



HISTORICAL NOTICE, 189 



other ; so that the energy of the waves of those particular chakxii 
qualities of light is converted into thermal vibrations of 
the medium and dispersed in all directions, while light of 
all other qualities, even though very nearly agreeing with 
them, is transmitted with comparatively no loss. 

•* 5. That Fraunhofer's double dark line D of solar and 
stellar spectra is due to the presence of vapour of sodium 
in atmospheres surrounding the sun and those stars in 
whose spectra it had been observed. 

" 6. That other vapours than sodium are to be found in 
the atmospheres of sun and stars by searching for sub- 
stances producing in the spectra of artificial flames bright 
lines coinciding with other dark lines of the solar and 
stellar spectra than the Fraunhofer line D." 

The idea then of Stokes which connected radiation with Explami- 
absorption, and at once read the riddle set by the sun 
and stars, was this : the light emitted by an incandescent 
vapour is due to the vibrations of its molecules, as a 
sound-note emitted by a piano-wire is due to the vibration 
of the wire. We have only to go into a room where there 
is a piano, and sing a note, to find that the wire which 
corresponds to our note will respond to it. Now, in the 
same way when light is passing through a vapour the 
molecules of which vibrate at any particular rate, they 
will be urged into their own special rate of vibration by 
the vibrations of the light which is passing through them, 
which correspond to that particular rate. Hence the light 
will, so to speak, be sifted, and the force it has exercised 
in impelling the particles in the interrupting vapour to 
vibrate will tell upon it ; and in this way, those particular 
vibrations which have had the work to do will be enfeebled. 
It is clear that the parts of the spectrum thus reduced 
in brilliancy will depend upon the vapour through which 
the light has passed. If sodium vapour be traversed, 
then the light corresponding to the bright lines of sodium 
will be enfeebled. 



190 



SOLAR PHYSICS. 



CHAP. XII. 

Stokes' 
statement. 



Anf^ftrbm^ 



Stokes first gave this dynamical illustration in the follow- 
ing words '?- " That a body may be at the same time a source 
of light giving out rays of a definite refrangtbility, and an 
absorbing medium extinguishing rays of that same re- 
frangibility which traverse it, seems readily to admit of a 
dynamical illustration borrowed from sound. We know 
that a stretched string which on being struck gives out a 
certain note (suppose its fundamental note), is capable of 
being thrown into the same states of vibration by aerial 
vibrations corresponding to the same note. Suppose now a 
portion of space to contain a great number of such stretched 
strings, forming thus the analogue of a ' medium.' It is 
evident that such a medium on being agitated would give 
out the note above mentioned; while, on the other hand, 
if that note were sounded in air at a distance, the incident 
vibrations would throw the strings into vibration, and 
consequently would themselves be gradually extinguished, 
since otherwise there would be a creation of vis viva. 
The optical application of this illustration is too obvious to 
need comment." 

Such was the theory which as I have shown was taught 
by Stokes prior to 1852, and by Thomson in his public 
lectures ever since. 

Our great physicist unfortunately did not publish it. I 
say unfortunately because valuable time has been lost : but 
not much, so far as publication or non-publication is con- 
cerned ; for, in 1853, the idea was published by the 
celebrated Angstrom.^ 

In his memoir, for the purpose of illustrating the absorp- 
tion of light, he made use of a principle already pro- 
pounded by Euler, in his Theoria bids et caloris, that the 
particles of a body, /// conseqtieucc of resonance^ absorb 
principally those ethereal undulatory motions which have 
previously been impressed upon them. He also endeavoured 



* Phil. Mag. vol. xix. p. 196. 

* "Optiska Undersokningar:" Trans. Royal Academy of Stockholm. 
1853. Translated \nPhii, Mag. 4th Scries, vol. ix. p. 237. 



HISTORICAL NOTICE. 191 



to show that a body in a state of glowing luat emits just the chap. xh. 
same kinds of light and heat which it absorbs under the 
same circumstances. He further undertook researches on 
the electric light, and stated that in many cases the Fraun- 
hofer lines were an inversion of the bright lines, which he 
observed in the spectrum of various metals.^ 

Early in 1859, Mr. Balfour Stewart independently dis- struHin, 
covered the law which binds together radiation and absorp- *^59- 
tion, establishing it experimentally as an extension of 
Provost's law of exchanges in the case of the heat rays, 
and generalizing his conclusion for all rays.* Stewart's 
reasoning is of a very ingenious and original nature. 

In October of the same year, 1859, Kirchhoff established 
experimentally the same law for the light rays. 

His first announcement, dated Heidelberg, 20th October, Alrch- 
1859. read before the Berlin Academy on the 27th,* must ^^-^^ '^^^' 
here be given in evtenso^ for it has certainly marked an 
epoch in solar physics. 

*' On the occasion of an examination of the spectra of 
coloured flames not yet published, conducted by Bunsen 
and myself in common, by which it has become possible 
for us to recognize the qualitative composition of complicated 
mixtures from the appearance of the spectrum of their 
blow-pipe flame, I made some observations which disclose 
an unexpected explanation of the origin of Fraunhofer's 
lines, and authorize conclusions therefrom respecting the 
material constitution of the atmosphere of the sun, and 
perhaps also of the brighter fixed stars. 

"Fraunhofer had remarked that in the spectrum of the 
flame of a candle there appear two bright lines which 
coincide with the two dark lines D of the solar spectrum. 
The same bright lines are obtained of greater intensity 
from a flame into which some common salt is put. I formed 

* See Phil, Mag. 4th Series, vol. xxiv. pp. 2, 3; Monatsbericht^ 1859, 
p. 662. ' Edinburgh Transactions, 1858-9. 

» See Translation by Professor Stokes in Phil. Mag. 4th Scries, voL 
xix. p. 195. 



SOLAR PHYSICS. 

. a solar spectrum by projection, and allowed the solar 
rays concerned, before they fell on the slit, to pass throi^h 
a powerful salt flame. If the sunlight were sufficiently 
reduced, there appeared in place of the two dark lines D 
two bright lilies ; if, on the other hand, its intensity sur- 
passed a certain limit, the two dark lines D showed them- 
selves in much greater distinctness than witliout the 
employment of the salt flame. 

" The spectrum of the Drummond light contains, as a 
general rule, the two bright lines of sodium, if the luminous 
spot of the cylinder of lime has not long been exposed to 
the white heat; if the cylinder remains unmoved, these 
lines become weaker, and finally vanish alt<^ether.' If 
they have vanished, or only faintly appear, an alcohol flame 
into which salt has been put, and which is placed between 




the cylinder of lime and the slit, causes two dark lines ol 
remarkable sharpness and fineness, which in that respect 
agree with the lines D of the solar spectrum, to shoir 
themselves in their stead. Thus the lines D of the solar 
spectrum are artificially evoked in a spectrum in which 
naturally they arc not present. 

" If chloride of lithium is brought into the flame of 
Hunsen's gas-lamp, the spectrum of the flame shows a very 
brijjht sharply defined line, which lies midway between 



HISTORICAL NOTICE. 193 

Fraunhofer's lines B and C. If, now, solar rays of mode- chap. xii. 
rate intensity are allowed to fall through the flame on the ~T^ 

,. - ,. , , . « . 1 . , Effect of 

silt, the line at the place pointed out is seen bright on a brighter 
darker ground ; but with greater strength of sunlight there ^'^^^ 
appears in its place a dark line, which has quite the same 
character as Fraunhofer's lines. If the flame be taken 
away, the line disappears, as far as I have been able to see, 
completely. 

'' I conclude from these observations, that coloured flames 
in the spectra of which bright sharp lines present them- 
selves, so weaken rays of the colour of these lines, when 
such rays pass through the flames, that in place of the 
bright lines dark ones appear as soon as there is brought 
behind the flame a source of light of sufficient intensity, 
in the spectrum of which these lines are otherwise wanting. 
I conclude further^ that the dark lines of the solar spectrum Solar lines 
which are not evoked by tlie atmosphere of the earth exist ^cJrZin 
in consequence of the presence^ in the incandescent aimo- knmvn 
s^iere of the sun, of those substances which in the spectrum ^ ^*^^^"' 
of a flame produce bright lines at the same place. We 
iliqr JMume that the bright lines agreeing with D in the 
l Uf CUuui of a flame always arise from sodium contained 
il|Ci| ; the dark line D in the solar spectrum allows us, 
tiMrcfore, to conclude that there exists sodium in the sun's 
atmosphere 

" In the course of the experiments which have at present 

been instituted by us a fact has already shown 

itself which seems to us to be of great importance. The 
Dnimmond light requires, in order that the lines D should 
come out in it dark, a salt-flame of lower temperature. 
The flame of alcohol containing water is fitted for this, but 
the flame of Bunsen's gas-lamp is not. With the latter 
the smallest mixture of common salt, as soon as it makes 
itself generally perceptible, causes the bright lines of sodium 
to show themselves " 

Immediately after the publication of this important 

o 



194 SOLAR PHYSICS. 



ex, 



CHAP. XII. note of Kirchhoff's, Stewart * explained, in extension of his 
former work on the theory of exchanges, why it was that 
a salt flame of lower temperature was required to darken 
the D lines, pointing out that it was a phenomenon ana- 
logous to that presented when a piece of ruby glass is 
Kuby glass heated in the fire. So long as the ruby glass is cooler than 
:perinu:nu ^^ coals behind it, the light given out is red because the 
ruby glass stops the green, the green light is therefore 
analogous to the line D which is given out by an alcohol 
flame into which salt has been put. Should however this 
ruby glass be of a much higher temperature than the coab 
behind it, the greenish light which it radiates overpowers 
the red which it transmits, so that the light which reaches 
the eye is more green than red. This is precisely ana- 
logous to what is observed when a Bunsen's gas flame with 
a little salt is placed in front of the Drummond light, when 
the line D is no longer dark but bright. 

Shortly afterwards Kirchhoff" independently explained 
his results on the same theory. 

In the next chapter we shall show how fruitful of result 
this experimental verification of Stokes* theory by Kirch- 
hoff'and Stewart, really effected by Foucault, has been, and 
how the new field of investigation thus opened up has been 
explored. 

Here we may content ourselves by pointing out how, 
by the light of modern science, a simple experiment, made 
by means of sodium vapour and a beam of sunlight, with 
the powerful aid of a little prism, gave us a tremendous 
increase of our knowledge about distant worlds — worlds 
so immeasurably remote that it seemed hopeless for men 
to try and grapple with them. 

» Sec " On the Theory of Exchanges and its recent Extension,** br 
Balfour Stewart, B.A., Reports, 1861. I quote here the passage 
Tvhirh relates to the connection between the heat and light rays :— 

** We come now to the subject of light ; and since radiant light and 
heat have been shown by Melloni, Forbes, and others, to possess very 
many properties in common, it was of course only natural to suppose 
that facts analogous to those mentioned should hold also with regard 
to light. One instance will at once occur, in which this analog}' i» 



HISTORICAL NOTICE, 195 



perfect. For, as all opaque bodies heated up to the same tempera- cjtap. xir. 

ture radiate the same description of heat, so aJso when their common " 

temperature is still further increased, they acquire a red heat, or a 
yellow heat, or a white heat, simultaneously. 

" The idea of applying these views to light had occurred indepen- 
dently to Professor Kirchhoff and myself; but, although Kirchhoff 
slightly preceded me in publication, it will be convenient to defer the 
mention of his researches till I come to the subject of lines in the 
spectrum. 

"In February i860, 1 communicated to the Royal Society of London 
a paper in which certain properties of radiant light were investigated, 
similar to those already treated of with respect to heat. In this paper 
it was mentioned that the amount of light radiated by coloured glasses 
is in proportion to their depth of colour, transparent glass giving out 
very little light ; also that the radiation from red glass has a greenish 
tint, while that from green glass has a reddish tint. 

*' It was also mentioned that polished metal gives out less light than 
tarnished metal, and that, when a piece of black and white porcelain 
is heated in the fire, the black parts give out much more light than 
the white, thereby producing a curious reversal of the pattern. 

" All these facts are comprehended in the statement that in a con • 
stant temperature the absorption of a particle is equal to its radiation, 
and that for every description of light. 

**It was also noticed that all coloured glasses ultimately lose their 
colour in the fire, as they approach in temperature the coals around 
them ; the explanation being, that while red glass, for instance, gives 
out a greenish light, it passes red light from the coals behind it, while 
it absorbs the green, in such a manner that the light which it radiates 
precisely makes up for that which it absorbs, so that we have virtually 
a coal radiation coming partly from, and partly through, the glass. 

"In another paper, communicated to the Royal Society in May of 
the same year, it is shown that tourmaline, which absorbs in excess the 
ordinary rays of light, also radiates, when heated, this description of 
light in excess, but that when the heated tourmaline is viewed against 
an illuminated background of the same temperature as itself, this 
peculiarity disappears." 



O 2 



Sodium. 



RESULTS OBTAINED BY THE OLD METHOD, 

I. KiRCHHOFF's Maps and list of Elements 

IN THE Sun. 

cHAP.xiii. In the note of Kirchhoff 's which was given almost in extenso 
in the last chapter, we saw the demonstration of the fact 
of the existence of sodium in a relatively cool atmosphere 
surrounding the sun. Now sodium has a very simple 
spectrum ; Kirchhoff was not long before he tested his 
generalization by the most complicated spectrum he could 
find. He took for this purpose the spectrum of iron, one 
of enormous complication, for, as we now know, the spec- 
trum is traversed by lines throughout its whole length, no 
less than 460 lines having been already mapped, and their 
positions are now thoroughly well known to us — as ^h'ell 
known as the position of any star in the heavens. Kirch- 
hoff tried the iron spectrum, and he found, absolutely 
corresponding in position and in width and darkness to 
the bright iron lines which he saw in this spectrum, black 
lines in the solar spectrum. He instantly convinced him- 
self and soon convinced the world, that he had experi- 
mentally established not only the fundamental fact, which 
we have already stated, that gases and vapours have 
the power of absorbing those very rays which they them- 
selves give out when in a state of incandescence, bat 
Iron. that iron as well as sodium was present in the sun. Kircb- 
hoflf went on with this magnificent work — ^which included 
the construction of the first map and catalogue of the 



RESULTS OBTAINED BY THE OLD METHOD, 


p 


197 




lines of the solar spectrum 
approaching completeness, 








CHAi'.xtn. 








which had ever been given 


s 










to the world ; a monument 












of industry which cost the 












illustrious physicist one of 












his eyes — until he had 












arrived at the conclusion 


o 




. 






that sodium, iron, calcium. 




J 


Cakium, 




magnesium, nickel, barium. 






1 


magH^ 




copper, zinc, [potassium ?] 




— ^"^^"^^^^^1 


1 


iJZm, 




were in the solar atmo- 






u. 


.'S^Z. 




sphere ; that the existence 






I 


aJdaita 




of cobalt there was doubt- 






i 


Ihtlul. 




ful, and that gold, silver. 






1 






mercury, aluminium, cad- 






1 






mium, tin, lead, antimony, 


p^ 




1 

•3 






arsenic, strontium, lithium, 




and silicon, were absent. 




^^^^^^^^ 


1 






II. Angstrom's M.\p 


^ 




f 






AND Ll.ST. 


H 




•3 






Kirchhoff, however, was 






not the only one at work 




g 






at the problem. I have 




__^^^^^M 


1 






already alluded to Ang- 




~^^^^^^H 


1 






strom. He, too, tike Kirch- 






\ 






hoff, was constructing a 


a 












map of the lines in the 




^^^^^B 


- 






solar spectrum, with the 




— ■ - ^^^^^s 








important difference that 




^^^^^^s 








whereas Kirchhoff's scale 
was an arbitrary one, Ang- 


" 


^^^H 


p 


' ~ -' ~ ^^^^^1 


strom's scale was based 


■ 


— ^^^^^^B 








upon the lengths of the light 


-! 










waves. Nor was this all. 


ft 




d~ 


- 



198 SOLAR PHYSICS. 



ciiAp.xiii. he also set himself to determine the wave lengths of 
the bright lines given out by the various gases and 
vapours in the electric arc. 

This he accomplished oy obtaining a spectrum by 
diffraction instead of by refraction. The way in which 
a spectrum is obtained by refraction has been already 
explained. In diffraction observations a grating is used, 
placed usually at right angles to the axis of the colli- 
mator ; this replaces the prism used when refraction 
is employed : the grating consisting of excessively fine 
parallel equidistant lines scratched on glass by means of 
a diamond and dividing engine ; in some cases 4000 or 
5000 or even 6000 lines are thus produced in an inch. 
Thetfif' The diffraction spectrum results from interference, and 
ifi^r/nim ^^^ S'**^t''^S was Suggested by Fraunhofer, who made the 
first observations of this nature by using fine wires. Its 
use was suggested by the fact that the two mirrors or 
the biprism ordinarily employed may be replaced by two 
narrow apertures. 

The wave length is determined by accurate measures of 
the angle between the observing telescope and the axis of 
the collimator, and of the distance between the scratches, 
the wave length being included in the formula' — 

sm o = n 
d 

Where S = observed angle, 

n — order of the spectrum, 

d = distance between lines in millimetres, 

X = wave length. 

The wave lengths of the principal Fraunhofer lines thu« 
determined are laid down in Angstrom's map of the solar 
spectrum,^ on a scale such that the wave length can be read 
off to a ten millionth of a millimetre (O.OOCXXXDI), and csti- 

1 For further details see *' Jamin*s Cours dc Physique^ t. iii., Po3'*^ 
et seq. 

' ** Rccherchcs sur 'e Spectre Sola ire," par R. J. Anj^strom. Spccirc 
normal du Solcil. Alias dc six planches. Upsala : W. Schultx. Berlin 
F. DiimmliT, 1869. 



RESULTS OBTAINED BY THE OLD METHOD. 199 



mated toa hundred millionth (o.ocxDOOOOi). The wave lengths chap.xiii. 
of some of the principal Fraunhofer lines are as follows : — 

A 000076C09 E 000052689 

B 0*00068668 F 000048606 

C 0-00065618 G 000043072 

D' ) 00005 8950 H' ) 000039680 

D" j 000058890 • H'' 1 000039328 

It will be seen from the above table that the wave length 
decreases from the red end of the spectrum to the violet ; 
in other words that the red waves are the longest and 
the violet waves the shortest in the spectrum ; and as the 
velocity of light is 298,000,000 metres, or 298,000,000,000, 
millimetres, or 29,800,000,000,000,000,000 hundred-mil- 
lionths of a millimetre per second, it is clear that the num- 
ber of waves (which number will vary with each wave) 

which fall on the eye per second can be determined. Thus 

^ 29,800,000,000,000,000,000 , .„. 

we have for A — i;Vwv^ ~ 392 billions No. of 

70009 waves per 

, , , _ ,_ 29,800,000,000,000,000,000 second of 

per second (roughly) For Hj ^o^ time of A 



= 7S7 billions per second (roughly). 

As the velocity of light is the same for all waves, it 
follows that the number of waves per second varies inversely 
as the wave length in each case, and that the number of 
each wave per second multiplied into its wave length must 
give us a constant quantity, namely the velocity. 

The first results^ of Angstrom's comparison of the wave- 
lengths of metallic vapours with the Fraunhofer lines 
added the possibility of strontium and aluminium being 
among the solar elements, and this observer thus allocated 
the principal Fraunhofer lines. 

H^ and H* due to Calcium. 
G „ Iron. 
F „ Strontium and Iron (uncertain). 

» Communicated to the Royal Academy of Stockholm, October 8, 
J 86 1. P/tr7. Mag.. S. 4, vol. xxiv., p. i. 



and of H^, 



200 SOLAR PHYSICS. 



CIIAP.XIII. 



b 


due to 


Magnesium 


and Iron. 


D 


99 


Sodium. 




C 


99 


Hydrogen. 




B 


tf 


Potassium, 





Thaim Angstrom later, in his Rectierches sur le Spectre Solaire^ 

z^ucTnd S^v^s more details, and his assistant Thalen* sums up 

alumin- the work at that time by rejecting zinc and aluminium 

adds mat ^^^"^ ^^ ^^^^ ^^ ^^'^'" ^^^ments as given by Kirclihoflf, and 

ganesc and adding manganese and titanium. 

titanium 
to the 

solar HI. FiZEAU'S SUGGESTION AS TO EFFECT OF 

Motion on Wave Length. 



tlements. 



There is a point of fundamental importance to solar 
physics connected with this method of wave length mea- 
surement : not only does it give us a spectrum in which 
the position of each line gives us its wave-length, but it also 
gives us a power of detecting motion of the vapours in the 
solar atmosphere if such exist, according to a principle first 
AfotioHof indicated by Fizeau, that if a light source moves to or 
^^oufc! ^^^^ tl^c eye with a velocity comparable with the velocit>- 
attended of light itself, then a change in the position of the lines 
tjlui/tke in the spectrum will be brought about. 
spectral M. Fizeau's researches in this direction are so little 
"toit!^ known in this country, that I give the account of them 
at length.* 

" Si un corps sonorc dmettant un son continu et toujoitrs iclcfitiqoe 
se mcut avec unc vitessc comparable k celle du son, les ondes sonores 
ne seront pas symdtriqucment disposes autour du corps sonore, comme 
cda \ lieu lorsqu'il est au repos ; mais elles seront plus rapprocbto 
Ics uncs des autres dans la region vers laquelle aura lieu le mouve- 
mcnt, et plus dloigndes dans la r^ion oppos^, pour un obser\*ateur 
plac^ en avant ou en arri6re du corps sonore ; le son sera done diffifrent ; 
plus aigu dans la premiere position, plus grave dans la secondc. 

'*Si Tobscrvatcur k son tour est supposd en mouvemcnt, le corps 
sonorc rcstant immobile, le r^sultat sera scmblable, mais la loi do 
phenom^ne est diffdrcnle. 

' " Longueurs d*onde des raies mdtalliques." Nova Acta. Upsab. 
3 Socidtd Philomathiquc do Paris. Extraits des proces-verbauxdes 
Sciences. 23 Dec, 1848. p. 81. 



RESULTS OBTAINED BY THE OLD METHOD. 201 



" En calculant les vitesses qui correspondent aux intervalles de la chap.xiii. 

gamme on trouve les nombres suivants : pour produirc une d^vation 

d'un demi'ion, le corps sonore doit avoir une vitesse par seconde de 
21*25, pour ^^ ^on majeur 37*8, pour la tierce 68, pour I'octave 170. 
Dans le cas du corps sonore immobile et pour obtenir les m^mes 
notes I'observateur doit avoir les vitesses, 22*6 : 42*5 : 85 ; et 340. Les 
sons 6tnis ou re^us, dans des directions diff(6rentes de celles du mouve- 
ment se calculent en projetant la vitesse sur la nouvelle direction. 

L'auteur donne la description d'un appareil qu'il a employ^ et au 
moyen duquel on peut vAnfier et d^montrer commoddment ces cu- 
rieuses propridt^ du son, dans le cas du mouvement du corps sonore. 
Cette appareil est fondd sur le principe des roues dent^s de M. Savart, 
mais la disposition est inverse. Au lieu de dents mobiles rencon- 
trant dans leur mouvement un corps dlastique fixe, c'est le corps dlas- 
tique qui est placd sur la circonfdrence d'une roue et qui recontre dans 
son mouvement des dents fixes places sur la concavity d'un arc ex- 
t^rieur immobile, Ton a ainsi un appareil fixe qui jouit de la propridtd 
d'emettre des sons difT^^rents dans chaque direction particuli^re. Pour 
une certaine vitesse de rotation par example on aura en avant le son 
fondamental, en arri^re I'octave, et toutes les notes de la gamme dans 
des directions intermddiaires. 

^£n appliquant ces considerations k la lumi^re on arrive k des 
cons^uences curieuses et qui pourraient acquerir de Timportance si 
Fexperience venait k les confirmer. Un mouvement tr^s rapide et 
comparable k la vitesse de la lumi^re, attribud au corps lumineux ou k 
I'observateur, aura pour effet d'altdrer la'longueur d'ondulation de tous 
les rayons simples qui composent la lumi^re regue dans la direction 
du mouvement Cette longueur sera augmentde ou diminude suivant 
le sens du mouvement Considdrd dans le spectre, cet effet se tra- 
duira par un diplacement des rates correspondant au changement de 
la longueur d'ondulation. 

" En calculant la valeur du ddplacement angulaire de la raie D dans 
le cas ou le corps lumineux aurait la vitesse de la plan^te Vdnus, le 
spectre dtant formd au moyen d'un prisme de flint de 60" on trouve 
2", 65. 

" Pour le cas ou I'observateur seul serait en mouvement et animd 
d'une vitesse dgale k celle de la Terre, on trouve 2", 25. 

" En supposant que Ton mesure les deviations doubles et que I'on 
sc place successivement dans des conditions ou les mouvements en 
question seraient de signe contraire, ces quantites peuvent ^tre qua- 
drupldes, et Ton a lo*, 6 et 9 pour les valeurs prdcddenles. 

** L'auteur termine en examinant si ces consequences pourront 6tre 
soumises k I'observation, et il pense que les difficultds ne sont pas 
tclles qu'on ne puisse esperer de les surmonter." 

To see the bearing of this let us suppose that one of 
the constituents of the solar atmosphere, let us say hydro- 
gen, while it was giving out its light was moving towards 
us at the rate of say 50 miles (= 80466 metres) a second. 
Now the wave length of one of the lines due to hydrogen — 



202 



SOLAR PHYSICS. 



CHAP.XIII 



F is, as we have seen, 000048606 at the normal velocity. 
Now with a higher velocity the number of crests per second 
reaching the eye muf,t be greater, and therefore the effective 
wave length must be shorter. In the spectrum this shorten- 
ing of the wave is indicated by the position of the h'ne F 
changing towards the violet — the region of shorter waves. 

If we supposed the hydrogen receding from the eye, 
then the position of the line would be changed tov/ards 
the region of longer waves — ie. towards the red. 

Let us suppose such a change to be observed. Suppose 
a change into a less refrangible region, say a change of 
the F line, the normal wave length of which is 000048606 
from that position to 000048716. Obviously the wave 
has been lengthened by the recession of the source of light 
^^'bftlfff' from the eye, and the amount of recession, about 39 miles 
a second, is measured by the increased length of wave, 
the difierence in the wave length bearing the same ratio 
to the total wave length as the difference in the velocity 
bears to the velocity of light.^ Similarly if we suppose a 
change to a more refrangible region, the wave has been 
shortened by the approach of the light source. 



Waves 
H^hent'i 
by reces- 
sion. 



Shortened 
by ap- 
proach. 



The 

Tellnric 

lines. 



IV. The Telluric Lines. 

At the same time that many of the lines in the solar 
spectrum were being shown to be due to the presence 
of vapours in the sun, others were found to be due to 
absorption by the atmosphere of the earth, a question 
first investigated by Brewster, later by Brewster and Glad- 
stone, and afterwards by Piazzi Smyth, who gave much 
attention to the problem in his famous " Astronomer's 
Experiment " on the Peak of Teneriffe. This fact was 
first suspected because certain lines were found to var}* 
with the height of the sun. They were very faint when 
the sun was high, and got thicker and darker as the sun 
ai)proached the horizon. 

* Sec Clerk Maxwell, Phil. Trans. i«$68, p. 532. 



To Janssen belongs the credit 
of determining to what consti- 
tuent of our atmosphere this 
absorption is due. In 1864 this 
eminent physicist was sent on a 
mission by the enlightened autho- 
rities of the French Academy of 
Sciences, to investigate this pro- 
blem, and among other results lie 
observed the spectrum of a large 
bonfire through a thickness uf 
atmosphere of 21,000 metres ovlt 
the Lake of Geneva. In this ex- 
periment he saw the well-known 
lines; though close to the fin.' 
there was no absorption whatevt-i 
On his return to Pans he was 
enabled to show by the follow 
ing beautiful experiment that the 
lines are due to the aqiicoiis 
vapour in our atmosphere. Ht- 
used a long iron cylinder (in 
fact a long lenfrth of gas-main, 
which the Paris Gas Company had 
placed at his disposal) and filled 
it with steam, taking precautions 
to keep the temperature high, and 
the glass ends transparent. At 
one end of this he placed a bright 
flame, at the other he observed 
the light by means of a spectro- 
scope after it had traversed the 
whole length of tube. He thus 
obtained a spectrum which was 
the exact equivalent of that 
which is superadded on the trti..* 
solar one, and which becomes 



BKIi^ofll 



fxfurimenl 

OH the ai- 
'orplion cf 




J 



204 SOLAR PHYSICS. 

CHAP. XIII. most marked when, the sun being low, there is the 
greatest possible thickness of our atmosphere and its 
contained aqueous vapour to give rise to a large amount 
of absorption. 

Brewster, Brewster and Gladstone, Piazzi Smyth, Janssen 
and Angstrom have all mapped this spectrum ; the most 
elaborate map we have, a reduced copy of which is given 
in the accompanying woodcut, is due to the latter. 

It follows, from an inspection of Angstrom's larger 
map, that nearly all the lines in the spectrum less re- 
frangible than C are due to absorption by the aqueous 
vapour of our planet. 

V. KiRCHHOFF AND AnGSTROM'S CONCLUSIONS AS 

TO THE Constitution of the Sun. 

By the investigations into the actual presence of elements 
in the solar atmosphere to which we have before alluded, 
it was made known to us that in a relatively cool atmo- 
sphere surrounding the sun, the elements named were in 
a state of vapour. The riddle of the sun was read to a 
certain extent, and Kirchhoff read it in this way. He 
Kirchhoff Said : — "There is a solid or a liquid something in the sun, 
^^herJis'a S*^*"S ^ Continuous spectrum, and around this there arc 
solid or vapours of sodium, of iron, of calcium, of chromium, of 
nucUusin barium, of magnesium, of nickel, of copper, of strontium, 
the sun of cobalt, and of aluminium ; all these are existing in 
conilnutms ^^ atmosphcre and are stopping out the sun*s light If 
spectnim. the sun were not there, and if these things were observed 
in an incandescent state, we should get exactly these 
bright lines from them." 

Kirchhoff further imagined that the visible sun, the sun 
which we see — and wc may take the sun as an example 
of every star in the heavens — was liquid. 

Now in the sun wc have, first, a shining orb, dimmed 
to a certain degree at the edge, and here and there, over 
the sun, we see what arc called spots. Kirchhoff wished, 



RESULTS OBTAINED BY THE OLD METHOD. 205 



not only to connect his discoveries with the solar atmo- chap.xhi. 
sphere, but was anxious to connect it with this dimming ^^^^ 
near the limb and the spots. He said that the solar atmo- roumUd by 
sphere, to which all the absorption lines were due, was ^J^^g 
really outside the sun, and formed the corona ; and that absorption, 
the dimming of the limb was due to the greater absorp- 
tion of this atmosphere, owing, of course, to the light 
of the sun travelling through a much greater thickness 
of atmosphere in reaching our eye from the limb than from 
the centre of the disc. Furthermore, he said that the And that 
sun-spK>ts, which astronomers, from the time of Wilson, J^'J^^ 
had asserted to be cavities, were nothing but clouds floating 
in this atmosphere of vapour. 

Such, then, is KirchhofTs theory of the sun. There is a 
something — Kirchhoff said it was a liquid — which gives us 
a continuous spectrum, and between our eye and that incan- 
descent liquid surface there is an enormous atmosphere, 
built up of vapours of sodium, iron, and so on ; and the 
reason that we get the dark lines is, that the molecules 
of the substances named absorb certain rays, those, namely, 
which they produce when they are in an incandescent state. 

Angstrom on his part arrived at the same, or nearly the Angstrom 
same, conclusion. He imagined that the Fraunhofer lines ^^^^^ 
originate for the most part in the photosphere, or the 
gaseous envelope which immediately surrounds the sun. 
On this he makes the following important remark : — 

" It has been urged as an objection to the hypothesis 
that the Fraunhofer lines belong principally to the sun, 
that they ought in that case to appear stronger and more 
distinct when the rays come from the edge, than when they 
come from the centre of the disc, which according to Forbes's 
observation, was not the case. Forbes's observation took 
place under the different stages of a solar eclipse, under 
which circumstances I conceive it would be very difficult 
to preserve the appearance of the spectrum accurately in 
the memory. I therefore considered that experiment to 
be worth repeating. I used for this purpose an optical 



2o6 SOLAR PHYSICS, 



sun. 



cHAPxiii. theodolite with two telescopes, one of which was furnished 
_~ T with a slit opening to admit the sunlight ; the height of the 
mmtson orifice was reduced to about 4 milltms, and the sun image 
the differ- ^yas projected upon it from a DoUond object-glass, of 3*02 
tons of meters focal distance. The diameter of the image thus 
^^^dOt f*^^"^^^ ^'^s 128 millims, and by allowing the rays from 
limb of the different parts of this image to fall successively upon the 
opening, it was easy to see whether the Fraunhofer Hncs 
underwent any change. Any very remarkable change 1 
could not discover. AH that I fancied I could remark was 
that the intensity of the spectrum light is somewhat less 
when the ray comes from the edge than when from the 
centre of the disc ; and this is evidenced by the circum- 
stance that the fainter Fraunhofer lines show themselves 
in the latter case comparatively stronger, whereas, when the 
light comes from the centre of the solar disc, the fainter 
lines will sometimes even totally disappear, while the 
stronger lines, as for example some of the iron lines, appear 
with correspondingly increased brilliancy ; as we know by 
Kirchhoff's experiments that an increased difference of 
intensity between the source of light and the absorbing 
gas is favourable to the distinctness of the lines in the 
gas spectrum, it would seem that this observation is not 
repugnant to what we already know concerning the absorb- 
ing power of gases." 

VI. Stonev's Conxlusions. 

Such then was the theory of the physical constitution of 
the sun propounded by Kirchhoff and Angstrom, as a 
result of their spectroscopic enquiries. I have now to refer 
to still another, based upon an examination of the solar 
spectrum which we owe to Mr. Stoney, a theory which iiill 
impress all who read it with its masterly treatment of a 
confessedly difficult subject.^ 

Mr. Stoney, far from agreeing with Kirchhoff that the 
spots are clouds, refers them like Wilson to a dark bo<ly 

' Proc. R.S. vol. xvii. p. i, ft seq. 



^ 



RESULTS OBTAINED BY THE OLD METHOD. 207 



within a concentric luminous film at a vast depth beneath chap.x iii. 
the surface of the sun's atmosphere, the part of that 
atmosphere above the photosphere being "enormously 
transparent to most of the rays which emanate from the 
shell of clouds." But Mr. Stoney does not stop here, he Theory of 
goes on to add that " the upper layers of the atmospheres of '^''^'^^• 
sodium, magnesium and hydrogen, are cooler than those of 
iron and calcium, that these again are cooler than the upper 
layers of the atmospheres of nickel, cobalt, copper and 
zinc ; " thus " we have evidence both that the atmospheres 
of the several gases extend to diflferent heights, and that the 
temperature increases from the surface of the solar atmo- 
sphere downwards. He next suggests that the probable 
order of the elements in the atmosphere beginning at the 
outer boundary depends upon their atomic weights. In this 
way we should have the atmosphere of hydrogen far over- 
lapping all the rest ; then, at a profound depth, sodium and 
magnesium reaching nearly to the same height, as the 
masses of their molecules arc nearly equal ; next, at a great 
distance further down, calcium ; then, in a group reaching 
nearly to the same height, chromium, manganese, iron, nickel 
and cobalt ; then, within a moderate distance of these, 
copper and zinc; and lastly, after a vast interval, barium. 

This theory led Mr. Stoney before the eclipse of 1868 
to predict, with a considerable approach to truth, some of 
the results which have been since obtained. 

"When examined through a spectioscope, adapted to an equatorial 
telescope, much of the light of the corona may be found to resolve 
itself into a multitude of bright lines, the brightest being coincident 
vTiXh. the faintest of Fraunhofer's lines. If this should prove to be the 
case, and if the observer could train himself to distinguish in the 
hurry and under such novel circumstance the lines of the different 
gases, it would even be possible to ascertain how high in the sun's 
atmosphere each reaches, by using a curved slit, and noting ihc moment 
at which each set of lines is obliterated by the advancing moon. This 
would be a determination of exceeding interest. The observations 
should commence immediately after the beginning of totality, and be 
kept up to the end of it, as it is only from situations close to the sun's 
disc that the brightest lines can come. 

" Directly outside the photosphere there lies a stratum of the sun's 
atmosphere, which is still hotter than the photosphere, and on the 



2o8 SOLAR PHYSICS. 



CHAP. XIII. outer boundary of this hot region there appears to be a shell of exces- 
sively faint cloud, part of which is to be seen in Mr. De la Rue's photo- 
graphs of the eclipse of i860. It probably extends the whole way 
round the sun ; it is, therefore, very desirable that this faint shell, 
which seems to lie at a distance of eight or ten seconds of space from 
the edge of the sun's disc, should be observed."* 

Mr. Stoney was led to the opinion that in the upper 
regions of the sun's sodium atmosphere, at which the re- 
versal of the lines takes place, the temperature is lower than 
that of a Bunsen's burner. The winged appearance of some 
lines, especially those of hydrogen, led Mr. Stoney to 
associate that appearance with a great quantity of the 
substance to which the lines were due; the absence of 
wings, as in the case of sodium, indicating a small quaih 
Absence of tity. The absence of nitrogen and oxygen from the sun 
nitrogen j^j ^q ^^ conclusion that compound bodies exist in the 
oxygen, sun. The reason they are not revealed in the spectrum, 
being that the masses of the molecules are too high to 
enable them to reach the cool parts of the sun's atmosphere.' 

Here then ends the long parenthesis, which has extended 
over five Chapters, in which I have attempted to show 
how Spectrum Analysis helps us in the study of Solar 
Physics. So far we have confined ourselves to the method 
employed by Kirchhoff which deals with the average spec- 
trum of the sun. The remaining part of this book will 
deal in the main with the results obtained up to the present 
time by the new method, one in which each minutest portion 
of the sun is examined separately. To this new method 
reference has already been made in Chap. viii. 

* Monthly Notices y vol. xxviii. p. 19. Nov. 1867. 
* Proc. R.S. vol. xvii. p. 33. 



SmST RESULTS OF THE NEW 
METHOD} 



B year 1^65 two very important memoirs, dealing c 

, the tcicicopic and photographic observations 

MiUted up to that time on the subject of solar 

were given to the world. One of them was 

»tely printed in this country; the other appeared in 

E Cemftis Reiidns of the Paris Academy of Sciences. 

I shall not detain you with a lengthened notice of these 

larkable papers. I shall merely refer to the explanation 

t in both of tiiem of the reason that a sun-spot appears 

—the very keystone of any hypothesis dcahng with 

fr physical constitution of the sun. 

Iv£nglish science, represented by Messrs. Dc la Rue, 

'art, and Loewy, said that a spot is dark because the 

r light is absorbed by a cool, non-luminous, absorbing 

uospbere, pouring down there on to the visible surface of 

e sun — in other words, on to the photosphere. 

,ch science, represented by M. Faye, said that a spot 
: because it is a hole in the photosphere, and the 
ibly luminous and therefore radiating interior gases of 

un are there alone visible. 
Now most of you will see in a moment that here was a 
ar issue, which probably the spectro-scope, and possibly 



I A lecture delivered at ihc KoyAl In< 
Fridiiy cicninE, May 38th, tSfio. 



1 of (irenl liri 



2IO SOLAR PHYSICS, 



cHAP.xiv. nothing else, could solve ; for the spectroscope is an instru- 
ment whose special niMer it is to deal with radiation and 
absorption. It tells us that the light radiated from dif- 
ferent bodies gives us spectra of different kinds, according 
to the nature of the radiating body, — continuous spectra 
without bright lines in the case of solids and liquids ; and 
bright lines, with or without continuous spectra, in the case 
of gases and vapours. It tells us also that absorption dims 
the spectrum throughout its length when the absorption b 
General general, and dims it here and there only when the absorp- 



anisdtC' ^jqj^ jg selective ; the well-known Fraunhofer lines being, 

trveabsorp' c \ c* 

tioH. you will readily see, an instance of the latter kind. So 
that we have general and selective radiation, and general 
and selective absorption. 

Now then, with regard to the English theory, if thcrt 
were more absorption in a spot than elsewhere, we might 
expect evidences of absorption ; that is, the whole solaf 
spectrum would be visible in the spectrum of a spot, but 
it would be dimmed, either throughout the length of tic 
spectrum or in places only. 

With regard to the French theory, having only radiating 
gaseous matter to deal with, we should, according to 
the then generally received idea, get bright lines only 
in the spot spectrum. 

Here, then, was a tempting opportunity, and one which I 
considered myself free to use ; for. although the spectro- 
scope had then been employed — and you all know hot 
nobly employed — for four years in culling secrets froa 
stars and nebul.ne, there was not, so far as I know, citbcf 
published or unpublished observation on the sun, tk 
nearest star to us. The field was therefore open for me. 
and I was not entering into another man's labour, whcn,ot 
the 4th of March, 1866, I attached a small spectroscope to 
my telescope, in order to put the rival theories to a teA 
and thus bring another power to bear on a question whidi 
had remained a puzzle since it was first started by Galiko 
some two and a half centuries ago. 



FIRST RESULTS OF THE NEW METHOD. 211 



What I saw I will describe more fully by and by. It is chap.xiv. 
sufficient here to mention that it was in favour of the 
English theory. There was abundant evidence of absorp- Resu/t. 
tion in the spots, and there was not any indication of 
gaseous radiation. 

Having then thus spectroscopically broken ground on Conception 
the sun, a very natural inquiry was how next to employ ^{Jl^ 
this extension of a method of research, the discovery of 
which Newton had called, nearly 200 years before, **the 
oddest, if not the most considerable, detection which hath 
hitherto been made in the operations of nature." 

There seemed one question which the spectroscope should 
now put to the sun above all others, and it was this : — 

'* Assuming this absorbing atmosphere to encircle the 
sun, in accordance with the general idea and Kirchhoff's 
hypothesis, what are those strange red flames seen appa- 
rently in it at total eclipses, jutting here and there from 
beyond the sun's hidden periphery, and here again hanging 
cloudlike.?" 

The tremendous atmosphere, which apparently the spec- 
troscope had now proved to be a cool absorbing one, was 
supposed to be indicated during eclipses by a halo of light 
called the "corona," in which corona the red flames are 
visible. Now, as the red flames are always observed to 
give out more light than the corona, they were probably 
hotter than it ; and reasoning thus on the matter with 
my friend Dr. Balfour Stewart one day, we came to 
the conclusion that they were most probably masses of 
glowing gas. 

Now, this being so, the spectroscope could help us, and 
in this way. 

The light from solid or liquid bodies, as you all I am Effi-ci of 
sure know, is scattered broadcast, so to speak, by the prism /^^7/?/w 
into a long band of light, called a continuous spectrum, solids and 
because from one end of it to the other the light is ^^"i^^'^'^ 

persistent. 

P 2 



SOLAR HHVSICS. 

The liiiht from gaseous and vaporous bodies, on ihe 
contrary, is most brilliant in a few channels ; it is husbandii, 
aiiiJ, instead of being scattered broadcast over a long band, 
is limited to a few lines in the band — in some cases to »■ 
VL-rv- few lines. 

Huiicc, if we iiave two bodies, one solid or liquid and 
the utlier gaseous or vaporous, which give out exactly 
equal amounts of light, then the bright lines of the latter 
will be brighter tiian those parts of the spectrum of the 
other to which they correspond in colour or rcfrangibility. 

Again, if the gaseous or vaporous substance gives out 
but few lines, then, although the light which emanates front 
it may be much less brilliant than that radiated by a solid 
iir liquid, the light may be so localized, and therefore 
intensified, in one case, and so spread out. and therefore 
diluted, in the other, that the bright lines from the feebl* 
in the spectroscope appear much brighler 
1 the corresponding parts of the spectrum of the more 
lustrous solid body. Now here comes a verj- important 




FIRST RESULTS OF THE NEW METHOD. 213 



said, that there was a possibility that if we could bring a chap.xiv. 
spectroscope on the field we might turn the tide of battle 
altogether, assuming the prominences to be gaseous, as the 
reflected continuous spectrum might be dispersed almost 
into invisibility, the brilliancy of the prominence lines 
scarcely suffering any diminution by the process. 

The first attempt was made in 1866, a Herschel-Browning Attemf^t to 
spectroscope being attached to my telescope, and the first ^Znes'/n 
and many succeeding attempts failed ; there was not dis- 1866 /f/Av/ 
persion enough to dilute the spectrum of the regions near 'l!f Tun^fl!. 
the sun sufficiently, and as a consequence the tell-tale cunt ins- 
lines still remained veiled and invisible. Nature's secrets 
were not to be wrested from her by a coup de main. 

The year 1868 brought us to the now famous eclipse, 
to see which scientific men hastened from all civilized 
Europe to India. To this eclipse and its results I need 
only refer, as they have already been dwelt on at some 
length in this theatre ; suffice it to say that in the eclipse 
the spectroscope did its duty, and that the gaseous nature 
of the prominences was put beyond all question. 

But there was a magnificent pendant to the eclipse, to 
which I must request your special attention. One of the 
observers, M. Janssen — a spectroscopist second to none — Juniscus 
the representative, in that peaceful contest, of the Academic 
des Sciences and of the Bureau des Longitudes, was so struck 
with the brightness of the prominences rendered visible by 
the eclipse that, as the sun again lit up the scene, and the 
prominences disappeared, he exclaimed, ** Jc jrctrrai ccs 
lignes la ! " and, being prevented by clouds from putting 
his design into execution that same day, he rose next 
morning long before the sun, and as soon as our great 
luminary had risen from a bank of vapours, he succeeded 
in obtaining spectroscopic evidence of the protuberances 
he had seen surrounding the eclipsed sun the day before. 
During the eclipse M. Janssen had been uncertain even i:s 
to the number of lines he had observed, but he now by this 
new method at his leisure determined that the promincncts 



oh-t'} 7 ij- 
t/'ors. 



SOLAR PHySICS. 



were built up of hydrogen, this fact being indicated by th 
presence of two bright lines corresponding to the darl 
lines C and F in the ordinary solar spectrum. 

Let me show you how this result was accomplished, b; 
throwing an enlat^ed phot<^raph of my telescope am 




■<™ ^ser- spectroscope on the screen. We have first the objcct-glas 

'^"■' of the telescope, to collect the sun's rays and to form a 

'miuctat. image of the sun itself on a screen. In this screen is n 

excessively narrow slit, through which alone light cai 

reach the spectroscope. This entering beam is grasped b; 

another little object-glass and transformed into a cylinder 

' Cylindrical, or nearly so, thai is, in ihc case of each pencil. 



FIRST KESUITS OF THE NEW METHOD. 

of lijrht containing rays of all colours, which is now ready ' 
for its journey through the prisms. In its passage through 
them it is torn by each succeeding prism more out of its 
path, till at last, on emerging, it crosses the path it took on 
entering, and enters the little telescope you see, thoroughly 
dismembered but not disorganized. 

Instead now of a cylinder of light containing rays of all 
colours, we have a cylinder of each ray which the little 
telescope compels to paint an image of the slit. Where 



rays are wanting, the image of the slit remains unpaintcd 
— we get a black line ; and when the telescope is directed 
to the sun, so that the narrow slit is entirely within the 
image of the sun, we get in the field of view of the little 
telescope a glorious coloured band with these dark lines 
crossing it. 

Of course it is necessary for our purpose to allow only - 

the edge of the sun to fall on the slit, leaving apparently 

a large portion of the latter unoccupied. What is seen, 

therefore,' is a very narrow band in the field of view of the 

' This refers lo obsfer\aiions with a radial slit. 



SO/.AR PHYSICS. 



telescope, and a large space nearly dark, as the dispersioi 
of the instrument is so great that the atmospheric light J: 




>'u. Ml,— Luc <J<^l.1l^ with radial slil. 

almost entirely got rid of, for a reason jo 
acquainted with. 




I. ll7.-l.iiK U,tty<;J.»»\HilhnMlialilil. 



Mr. Ladd v.ill now show you on the screen what is sen 
when the .slit reaches a prominence. First a line in the 



^ 



FIUST RESULTS OF THE AEW METHOD. 



d, very obvious and brilliant, next a more delicate line t 




the yellow, then another in the green, and two others in 
le violet ; ail these lines, with the exception of the yellow 



iiUiiiiliih lilfilllHlilli 



Fi.. B9,-L,r..C, 



are in the posit io 
rdrogen. 



:ciipitd by kn<>nn lines of 



2i8 SOLAR PHYSICS, 



cHAP.xiv. As the height of these bright lines must vary with the 
height of the prominences, and as the lines will only be 
visible where there is any hydrogen to depict them, it is 
obvious that the form of the prominences may be deter- 
mined by confining the attention to one line, and slowly 
sweeping the slit over it. 
First The first fruits, then, of this new method of working with 

fruits. ^^ uneclipsed sun was to tell us the actual composition of 
the prominences, and to enable us to determine their shapes 
and dimensions. 

For the next steps you must permit me to refer more 
particularly to my own observations. 
Thf Chro- When I was first able to obtain results in this country 
mosp ere. similar to those previously obtained by M. Janssen, though 
unknown to us, my instrument was incomplete ; when 
other adjustments had been added by Mr. Browning, I 
found that at whatever part of the sun*s edge I looked, I 
could not get rid of the newly-discovered lines. They 
were not so long as I had seen them previously, but there 
they were, not to be extinguished, showing that for some 
5,000 miles in height all round the sun there was an enve- 
lope of which the prominences were but the higher waves. 
This envelope I named the *' Chromosphere," as it is the 
region in which all the variously coloured effects are seen 
in total eclipses, and because I considered it of importance 
to distinguish between its discontinuous spectrum and the 
continuous one of the photosphere. And now another 
Thickening fact camc out. The bright line F took the form of an 
line. arrow-head (see Fig. Z^), the dark Fraunhofer line in the 
ordinary spectrum forming the shaft, the corresponding 
chromospheric line forming the head ; it was broad close 
to the sun's edge, and tapered off to a fine point, an 
appearance not observed in the other lines. 

Nature is always full of surprises, and here was a sur- 
prise and a magnificent help to further inquiry lurking 
in this line of hydrogen! MM. Pliicker and Hittorf had 



FIRST RESULTS OF THE NEW METHOD. 219 



already recorded that, under certain conditions, the green chap.xiv. 
line of hydrc^en widened out ; and it at once struck ine 
that the " arrow-head " was nothing but an indication of 
this widening out as the sun was approached. 

I will now, then, for one moment leave the observatory 
work to say a word on some results recently obtained by 
Dr. Frankland and myself, in the researches on the radi- 
ation and absorption of hydrogen and other gases and 
vapours, upon which we have for some time been engaged. 

First, as to hydrogen : what could laboratory work tell 
us about the chromosphere and the prominences ? 
It was obviously of primary importance — 

1. To determine the cause to which the widening of the 
F line was due. 

2. To study the hydrogen spectrum very carefully under 
varying conditions, with a view of detecting whether or 
not there existed a line in the orange. 

We soon came to the conclusion that the principal, if v Hfieis 
not the only cause of the widening of the F line was J^^f^*'^^ 

^ ^ by pressure. 

pressure. 

Having determined, then, that the phenomena presented 
by the F line were phenomena depending upon and indi- 
cating varying pressures, we were in a position to determine 
the atmospheric pressure operating in a prominence, in 
which the red and green lines arc nearly of equal width, 
and in the chromosphere, through which the green line 
gradually expands as the sun is approached. 

With regard to the higher prominences, we have obtained //iii^her 
evidence that the gaseous medium of w^hich they are com- ^^^^7 
posed exists in a condition of excessive tenuity ; and that tenuous. 
even at the lower surface of the chromosphere, that is, on 
the sun itself, in common parlance, the pressure is very far 
below the pressure of the earth's atmosphere. 

Now I need hardly point out to you that the deter- 
mination of the above-mentioned facts leads us necessarily 



220 



SOLAR PHYSICS. 



CHAF.XIV. 



I^earing on 

Alrchhoff's 

theory. 



An incan- 
deicent at- 
mosphere 
surrounds 
the photo- 
sphere. 



to several important modifications of the received theory 
of the physical constitution of our central luminary — ^the 
theory which we owe to Kirchhoff, who based it upon his 
examination of the solar spectrum. According to his 
hypothesis, the photosphere itself is either solid or liquid, 
and it is surrounded by an extensive cool and non-luminous 
atmosphere composed of gases and the vapours of the 
substances incandescent in the photosphere. 

We find, however, instead of this compound cool and 
non-luminous . atmosphere outside the photosphere, one 
which is in a state of incandescence, is therefore luminous, 
and which gives us merely, or at all events mainly, the 
spectrum of hydrogen ; and the tenuity of this incandescent 
atmosphere is such that it is extremely improbable that 
any considerable atmosphere, such as the corona has been 
imagined to indicate, exists outside it. 

Here already, then, we find the "cool absorbing atmo- 
sphere" of the theorists terribly reduced in height, and 
apparently much more simple in its composition than had 
been imagined by Kirchhoff and others. Dr. Frankland 
and myself have shown separately — 

1. That a gaseous condition of the photosphere is quite 
consistent with its continuous spectrum, whether we regard 
the spectrum of the general surface or of spots. The 
possibility of this condition has also been suggested by 
Messrs. De la Rue, Stewart, and Loewy. 

2. That a sun-spot is a region of greater absorption. 

3. That when photospheric matter is injected into the 
chromosphere, we see bright lines. 

4. That there are bright lines in the solar spectrum 
itself. 

All these are facts which indicate that the absorption to 
which the reversal of the spectrum and the Fraunhofer lines 
are due takes place in the photosphere itself or extremely 
near to it^ instead of in an extensive outer absorbing 

> I have italicised this passage in 1873, as some critics of my work 
have overlooked it. 



nUST RESULTS OF THE NEW METHOD. 



atmosphere. And this con- 
clusion is strengthened by 
the consideration that other- 
wise the newly- discovered 
bright lines of hydrogen 
should themselves show traces 
of absorption on KirchholT's 
theory ; but I shall show you 
presently that, so far from this 
being the case, they appear 
bright actually in the very 
centre of the disc, and, more- 
over, the vapours of sodium, 
iron, magnesium, and barium 
are often bright in the chro- 
mosphere, showing that the> 
would always be bright theri^ 
(/ the vapours xvcre alivays 
present, as they should be on 
Kirchhoff's hypothesis ; si. 
that we may say that the 
photosphere //«j the chromo- 
sphere is the real atmosphere 
of the sun, and that the sun 
itself is in such a state of fer- 
vid heat that the actual outer 
boundary of its atmosphere, 
i.e. the chromosphere, is in a 
state of incandescence. 

With regard to the line in 
the orange I have nothing yet 
to tell. Dr. Frankland and 
myself are at the present mo- 
ment working upon it. 

I have next to take you a 
stage lower into the bowels, 





SOLAR FHVSfCS. 



ConvKth. 
Iht al«ui- 



not of the earth, but of the sun. As a rule, the chromo- 
sphere rests conformably, as geologists would say, on the 
photosphere, but the atmosphere (as I have just defined 
it) is tremendously riddled by convection currents ; and 
where these are most powerfully at work, the upper layers 
of the photosphere are injected inta the chromosphere. 
Thus I have observed the lines due to the vapours of 
sodium, magnesium, barium, and iron in the spectrum of 
the chromosphere, appearing there as very short and very 
thin lines, generally much thinner than the black lines due 
to their absorption in the solar spectrum. 

These injections are nearly always accompanied by the 
strangest contortions of the hydrogen lines, of which more 
presently. Sometimes during their occurrence the chromo- 
sphere seems full of lines, those due to the hydrogen 
towering above the rest. 






Atlimpl I 



At the same time we have tremendous changes in the 
prominences themselves, which I have recently been able 
to see in all their beauty. I attempted to accomplish this 
in the first instance by means of an oscillating slit ; but 
hearing that Mr. Huggins had succeeded in doing the same 



FIRST RESULTS OF THE NEW METHOD. 223 



thing by means of absorptive media, using an open slit, chap.xiv. 
it struck me at once that an open slit was quite sufficient, 
and this I find to be the case. By this method the smallest 
details of the prominences and of the chromosphere itself 
are rendered perfectly visible and easy of observation, 
and for the following reason. As you already know, the 
hydrogen Fraunhofer lines (like all the others) appear dark 
because the light which would otherwise paint an image of 
the slit in the place they occupy is absorbed ; but when we 
have a prominence on the slit, there is light to paint the 
slit ; and as in the case of any one of the hydrogen lines 
we are working with light of one refrangibility only, on 
which the prisms have no dispersive power, we may con- 
sider the prisms abolished. Further, as we have the pro- 
minence image coincident with the slit, we shall see it as 
we see the slit, and the wider we open the slit the more of 
the prominence shall we see. We may use either the red, Any hy- 
or green, or blue light of hydrogen for the purpose of jif^l^^^f^g 
thus seeing the shape and details of the prominences ; how usMtosee 
far the slit may be opened depends upon the purity of the ' ^^^^^' 
sky at the time. 

I have been perfectly enchanted with the sight which 
my spectroscope has thus revealed to me. The solar and 
atmospheric spectra being hidden, and the image of the 
wide slit and the part of the prominence under obser- 
vation alone being visible,^ the telescope or slit is moved 
slowly, and the strange shadow-forms flit past, and arc 
seen as they are noticed in eclipses. Here one is reminded, 
by the fleecy, infinitely delicate cloud-films, of an English 
hedge-row with luxuriant elms ; here of a densely inter- 
twined tropical forest, the intimately interwoven branches 
threading in all directions, the prominences generally ex- 
panding as they mount upwards, and changing slowly, 
indeed almost imperceptibly. 

It does not at all follow that the largest prominences 

' This was accomplished by inserting a diaphragm with a small 
square aperture in the centre. 



SOLAR PHYSICS. 




. are those in which the intensesE action, or the must rapid 
change, is going on — the action as visible to us bcinj; 
generally confined to the regions just in, or above, the 
chromosphere ; the changes arising from violent uprush or 
rapid dissipation — the uprush and dissipation representing 
the birtli and death of a prominence. As a rule, the 
attachment to the chromosphere is narrow and is not often 
single ; higher up, the stems, so to speak, intertwine, and 
the prominence expands and soars upward until it is losi 




in delicate filaments, which are carried away in floatioe 
masses. 

Since last October, up to the time of trj'ing the method 
of using the open slit, I had obtained evidence of con- 
siderable changes in the prominences from day lo day. 
With the open slit it is at once evident that changes on 
the small scale arc continually going on ; but it was oiJ}' 




FIRST RESULTS OF THE NEW METHOD. 



on the 14th of March that I observed any change at all ' 
.comparable in magnitude and rapidity to those already 
recorded by M. Janssen. 

About gh. 45m. on that day, with the slit lying nearly 
along (he sun's edge instead of across it as usual, I observed 
a fine dense prominence near the sun's equator, on the 
eastern limb, with signs of intense action going on. At 
loh. 5oni„ when the action was slacivening, I opened the 




slit and saw at once that the dense appearance had all dis- 
appeared, and cloud-like filaments had taken its place. The Changn in 
first sketch, now exhibited (Fig. 92), embracing an irregular ^^X 
prominence with a long perfectly straight one, was finished 
at 1 ih. 5m., the height of the prominence being 1' 5", or 
about 27,000 miles. I left the Observatory for a few 
minutes, and on returning, at iih. 15m., I was astonished 
to find that the straight part of the prominence had en- 
tirely disappeared ; not even the slightest rack appeared 




J 



SOLAR PHYSICS. 



cHAP.xiv. in its place. Whether it was entirely dissipated, or whether 
~ parts of it had been wafted towards the other part, I do 
not know, although I think the latter explanation the 
more probable one, as the other part had increased. 

So much, then, for the chromosphere and the prouii- 
nences, which I think the recent work has shown to be 
the last layer of the true atmosphere of the sun. I 

shall now invite your attention to spots. 

Thicktn- Now, as a rule, precisely those lines which are injected 

'"^if'tfe- '"'"^ ^^^ photosphere by convection currents are most 

trumofa thickened in the spectrum of a spot, and the thickening 

'^' increases with the depth of the spot, so that I no longer 




regard a spot simply as a cavity, but as a place in whidi 
principally the vapours of sodium, barium, iron, and 
magnesium occupy a lower level than they do ordinarily 
ill the atmosphere. 

I have told you before, that when these lines are observed 
in the chromosphere, they are usually thinner than their 
Fraunhofer lines. 

I will now show a photograph of a spot-spectrum on 
the screen. You will see a black band running across thi; 
ordinary spectrum ; that black band indicates the gcneol 
absorption which takes place in a sun-spoL Now mark tlw 
behaviour of ihe Fraunhofer lines ; see how they widen ^ 
they cross the .spot, putting on a sudden blackness afld 



FIRST RESULTS OF THE NEIV METHOD. 227 

width in the case of a spot with steep sides, expanding chap.xiv. 
gradually in a shelving one. The behaviour of these lines 
is due to selective absorption. 

We have, then, the following facts : mark them well : — 

1. The lines of sodium, magnesium, and barium, when 
observed in the chromosphere, are among those which are 
thinner than their usual Fraunhofer lines. 

2. The lines of sodium, magnesium, and barium, when 
observed in a spot, are among those which are thicker than 
their usual Fraunhofer lines. 

They show, I think, that a spot is the seat of a downrush 
or downsinking. 

Messrs. De la Rue, Stewart, and Loewy, who brought 
forward the theory of a downrush before my observations 
of an actual downrush were made in 1865, at once sug- Dmunrmh 
gested as one advantage of this explanation, that all the ^^Jf^^n 
gradations of darkness, from the faculae to the central in 1865. 
umbra, may be supposed to be due to the same cause ; 
namely, the presence to a greater or less extent of a 
relatively cooler absorbing atmosphere ; thus suggesting as 
one cause of the darkening of a spot — 

1. The general absorption of the atmosphere, thicker here 
than elsewhere, as the spot is a cavity. 

To which the spectroscope added in 1866, as you 
know — 

2. Greater selective absorption. 

I have Dr. Frankland's permission to exhibit an experi- 
ment connected with our researches on absorption which 
will show you that this increased selective absorption can 
be fairly grappled with in our laboratories. I will show you 
on the screen the absorption line due to sodium vapour, in Absorption 
one part as thin as it is in the ordinary solar spectrum ; in ^^^^^* 
another, almost if not quite as thick as it appears in a 
spot ; and I accomplish this result in the following way : — 
Here I have an electric lamp, and by means of this slit 
I only permit a fine line of light to emerge from it ; here 
the beam passes through a bisulphide of carbon prism, and 

Q 2 



228 SOLAR PHYSICS. 



cHAP.xiv. there you see on the screen the glorious spectrum, due to 
the dismemberment of the fine line of polychromatic light. 
Mr. Pedler will now place a glass tube containing metallic 
sodium, sealed up with hydrogen, in front of the slit, and 
will heat it with a spirit lamp. 

As the sodium vapour rises you see the dark line of 
absorption make its appearance as an extremely fine Hne, 
and finally you see that the light which traverses the upper 
layer of the sodium vapour scarcely suflers any absorption— 
the line is thin ; while, on the contrary, the light which has 
traversed the lower, denser layers has suffered tremendous 
absorption : the line is inordinately thick, such as we sec it 
in the spectrum of a spot. 

So much, then, for the selective absorption. My reccot 
observations, to which I will shortly draw attention, shov, 
I think, that it is of g^eat importance, especially in con- 
nection with the fact that the passage from the penumbra 
to the umbra is generally less gradual than that from the 
photosphere to the penumbra. You see now how much is 
included in the assertion that the photosphere is gaseous. 

You are all, I know, familiar with that grand genendixa- 
tion of KirchhofTs, by which he accounted for the Fraoa- 
hofer lines. 

If we have a gas or a vapour less luminous than another 
light-source, and view that light-source through the gas or 
vapour, then we shall observe absorption of those paiticabr 
rays which the gaseous vapour would emit if incandcsool 

Let us confine our attention to the hydrogen Fraunbofcr 
lines. 

When I observe the chromosphere on the sun's Eobi 
with no brighter light-source behind it, I obsenne te 
characteristic lines bright. But when I observe tbcsi « 
the sun itself — that is, when the brighter sun is oa de 
other side of the hydrogen envelope, then, as a mlc to 
function is reduced — is toned down — the envelope 
an absorber — the lines are observed black. 



FIRST RESULTS OF THE NEW METHOD. 229 



Now what must we conclude when I tell you that, at the chap.xiv. 
present time, it is almost impossible to observe the sun for 
an hour without observing the hydrogen lines, every now //Wj fem 
and then, bright upon the sun itself! bright upon 

Not only are the lines observed bright, but it would Usdf, 
appear that the strongly luminous hydrogen is carried up 
by the tremendous convection currents at different pres- 
sures; and under these circumstances the bright line is 
seen to be expanded on both sides of its normal position. 
Moreover, at times there is a dim light on both sides the 
black line, and the line itself is thinned out, showing that, 
although there is an uprush of strongly luminous material, 
the column is still surmounted by some less luminous 
hydrc^en, possibly separated from the other portion, which 
still performs the functions of an absorber. This seems 
established by another fact, namely that at times the lines, 
still black, expand on both sides, as if, in fact, in these 
regions there were a depression in the chromosphere ; you 
already know that the pressure is greater at the base of the 
chromosphere than at the summit 

For this reason it is best to observe these phenomena by -^'"^ ^* 
means of the green line, which expands in a more decided sithe to 
manner by pressure than does the red. pressure 

I now come to a new field of discovery opened out by 
these investigations, a branch of the inquiry which I fear 
you will consider more startling than all the rest — a branch, 
however, which I have had many opportunities of study- 
ing, and which has required me to move with the utmost 
caution. I allude to the movements of the hydrogen Moztmmis 
envelope and prominences at which I have before hinted. moj/Lre. 

Anyone who has observed the sun with a powerful 
telescope, especially in a London fog — all too great a rarity 
unfortunately for such work — will have been struck with 
the tremendous changes observed in spots. Now, change 
means movement; and as spot phenomena occur imme- 
diately below the level of the chromosphere, we may easily 



r 



SOLAR pi/ysics. 

imagine that the chromosphere and its higher waves, the 
prominences, will also partake of the movements, be they 

up- or down-rushes, cyclones, or merely lateral motions. 
I have thrown on the screen a photograph of a drawing 
of a sun-spot observed under the clear sky of Rome by 
Father SecchJ — a drawing I regard as a must faithful coun- 
terpart of nature. 

You see how the photosphere is being driven about and 




contorted ; how here it seems to be torn to ribbons by the 
action of some tremendous force, how here it is dr^gcd 
down and shivered to atoms. 

The spectroscope enables us to determine the velodlie* 
; of these movements with a considerable approach to 
accuracy ; and at times they are so great that 1 aru almost 
afraid to mention them to you. 

Let me first endeavour to give you an idea how thi* 
result is arrived at, and I must here beg your indulgence 
for a gross illustration of one of the most supremely 
delicate of Nature's operations. 



FIRST RESULTS OF THE NEW METHOD. 231 

Imagine a barrack out of which is constantly issuing chap.xiv. 
with measured tread and military precision an infinite 
number of soldiers in single or Indian file ; and suppose 
yourself in a street seeing these soldiers pass. You stand 
stilly and take out your watch, and find that so many pass 
you in a second or minute, and that the number of soldiers, 
as well as the interval between them, is always the same. 

You now move slowly towards the barrack, still noting 
what happens. You find that more soldiers pass you than Change of 
before in the same time, and, reckoned in time, the interval j^^^'^ 
between each soldier is less. motion of 

You now move still slowly from the barrack, ue, with the ^reat^- 
soldiers. You find that fewer soldiers now pass you, and plained. 
that the interval between each is longer. 

Now suppose yourself at rest, and suppose the barrack 
to have a motion now towards you, now from you. 

In the first case the men will be payed out, so to speak, 
more lapidly. The motion of the barrack-gate towards 
you will plant each soldier nearer the preceding one than 
he would have been if the barrack had remained at rest. 
The soldiers will really be nearer together. 

In the second case it is obvious that the interval will be 
greater, and the soldiers will really be further apart. 

So that, generally, representing the interval between 
each soldier by an elastic cord, if the barrack and the eye 
approach each other by the motion of either, the cord will 
contract ; in the case of recession, the cord will stretch. 

Now let the barrack represent the hydrogen on the sun, 
perpetually paying out waves of light, and let the elastic 
cord represent one of these waves ; its length will be 
changed if the hydrogen and the eye approach each other 
by the motion of either. 

Particular wave-lengths with the normal velocity of light 
are represented to us by different colours. 

The long waves are red. 

The short waves are violet. 

Now let us fix our attention on the green wave, the 



232 



SOLAR PHYSICS. 



CHAP. XIV. refrangibility of which is indicated by the F line of hydro- 
gen. If any change of wave-length is observed in this 
line, and not in the adjacent ones, it is clear that it is not to 
the motion of the earth or sun, but to that of the hydrogeo 
itself and alone that the change must be ascribed. 

If the hydrogen on the sun is approaching us, the waves 
will be cruslied together ; they will therefore be shortened, 
and the light will incline towards the violet, that is, towards 
the light with the shortest waves ; and if the waves arc 
shortened only by the j— -l-^^^th of a millimetre, we can 
detect the motion. 

If the hydrogen on the sun is receding from us, the 
waves will be drawn out; they will therefore be longer, and 
the green ray will incline towards the red. 



Conditions 

of ap- 
proach or 
recess. 



I must next point out, that there are two different 
circumstances under which the hydrogen may approach or 
recede from the eye. 

I have here a globe, which we will take as representing 
the sun. Fix your attention on the centre of this globe;* 
it is evident that an uprush or a downrush is necessary to 
cause any alteration of wave-length. A cyclone or lateral 
movement of any kind is powerless; there will be no 
motion to or from the eye, but only at right angles to the 
line of sight. 

Next fix your attention on the edge of the globe— the 
limb, in astronomical language ; here it is evident that an 
upward or downward movement is as powerless to alter the 
wave-length as a lateral movement was in the other case, 
but that, should any lateral or cyclonic movement occur 
here of sufficient velocity, it might be detected. 

So that we have the centre of the disc for studying 
upward and downward movements, and the limb for 
studying lateral or cyclonic movements, if they exist 

If the hydrogen-lines were invariably observed to 
broaden out on both sides, the idea of movement would 

* I,e, the middle point of the hemisphere turned to each observer. 



FIRST RESULTS OF THE NEW METHOD. 



require to be received with great caution : we might be in chap.xiv 
presence of phenomena due to greater pressure, both when „. 
the lines observed are bright or black upon the sun ; but dutieprn 
when they widen out, sometimes on one side, sometimes ^^^/jlj" 
on the other, and sometimes on both, this explanation lameoH 
appears to be untenable, as Dr. Frankland and myself in ^^^^ i'^ 
our researches at the Collie of Chemistry have never 
failed to observe a widening out, equally or nearly so, on 
both sides the F line when the pressure of the gas has 
been increased. 



You see now on the screen (Fig. 96) a diagram showing the 
strange contortions which the F hydrogen hne undergoes at 




Fig. 96. — Conlonioiu of F line on diic. i ai . 

ruAhei Auociaud with hydrogen at rcii. 

the centre of the sun's disc. Not only have we the line 
bright, as I have before told you, but the dark one is 
twisted in places, generally inclining towards the red ; and 
often when this happens we have a bright line on the 
\'iolet side. You see it (Fig. 97), sometimes, stopping short 
of one of the small sun-spots; swelling out prior to disap- 
pearance ; invisible in a facula between two small spots ; 
changed into a bright line, and widened out on both sides 
two or three times in the very small spots; becoming 
bright near a spot, and expanding over it on both sides ; 
very many times widened out near a spot, sometimes 
considerably, on the less refrangible side; and, finally, 
extended as a bright line without any thickening over a 
small spot. 



SOLAR PHYS/CS. 



met/emtnt. 



Now the Other Fraunhofer lines on the diagram (Fig. 98) 
may be looked upon as so many milestones telling us with 
what rapidity the uprush and downrush take place; for 
these twistings are nothing more or less than alterations of 
wave-length, and thanks to Angstrom's map we can map 
out distances along the spectrum from F in -foTroVooo*^ 
of a millimetre from the centre of that line ; and we know 
that an alteration of that line mooVooo *^ """* ^^^'^'^^ ^^ 
violet means a velocity of 38 miles a second towards the 
eye, i.e. an uprush ; and that a similar alteration towards 




the red means a similar velocity from the eye, f>, a down- 
rush. The fact that the black line inclines to the red sbowi 
that the less bright hydrogen descends; the fact that the 
bright line — where both are visible side by side — indines 
to the violet, shows that the more vivid hydrogen ascends; 
and the alteration of wave-length is such that 20 miles a 
second is very common. 

Now, observations of the hiteral motions at the limb are 
of course made by the chromospheric bright lines seen 
I beyond the limb. Here the velocities are very much more 
startling ; not velocities of uprush and downrush, as yoo 
now know, but swinging and cyclonic motions of tbe 
hydrogen. 

I will first show you a cyclone observed on the Mtk 



FIRST RESULTS OF THE NEiV METHOD. 



of March, but before 1 do so let me make one remark. 
Although the sUt used is as narrow as I can make it, let 
us say siuth — I have not measured it — of an inch, a strip 
of this breadth, of the sun's image, is something consider- 
able, as the glorious sun himself is painted by my object- 
glass only about -94 inch in diameter, so that after all the 
slit lets in to be analysed a strip some 1,800 miles wide. 

Now, suppose we have a cyclone of incandescent hydro- 
gen some 1,500 miles wide tearing along with a very rapid 
rotatory motion, it is clear that all this cyclone could fall 
within the slit ; and that if the rotatory motion were suffi- 
ciently rapid, the spectroscope should separate the waves 
which are carried towards us from those which are receding. 



Site of the 
porthn rf 




It does this : as you sec, we have an alteration of wave- 
length both towards the red and violet, amounting to 
something like 40 miles a second. Now it should be clear 
to you that, by moving the slit first one way and then the 
other, we maj' be able to bring it in turn to such positions 
that only the light proceeding from either side of the 
cyclone can enter it. Then we shall have changes of wave- 
length io one direction only, in each case precisely as you 
see was observed. 

Now, let us suppo.se that, instead of a cyclone, we have 
a motion of some portions of the prominence towards the 
eye ; and that, moreover, the rate of motion varies exces- 




p-.rT,i:ii. af zta pr^munsnz: a: 7=e »-iL ^-.i b* »: aJiiJiMo 
-jf v-zvt-ttsizzL IS iriyh: ine viL jk it i. hns *t:a lie 
-;r-rj-p-jtijjir •■=a-r ant e 3it jairmat. Tas p?rion 
rijin-:!!^ iLiKSirdi lie rpt aOTPTi-s:. rf. p»^ a« ir aliera- 

pi-ii:Tj:c t: ^r^isi ibt Iitt=n3in9i& m>sju££ :i' id; by my 
i>^*!:=r;.-i=g^ -jz. zht rnr "wa-arr vies SI zimss lac F liae 
vti tr-plt -a; Kcrsme aksiKJDti a -vrnFC-^eagrii being 
!>j'^ ^hic r^i in:cii>£ :r :Juc par: oc die pcunmesoe giving 
tbt =i:*5: ±x:ztsDi: g"--r;-i.-«r x -wzve-Jeapb aua faat-c 
£x^«£if^ : 2: silies pc secosil if ve aic an ■■— p"'"* these 




thcnomena b}' the only koottii possible cause which U 
open to us. 

liy moving the slit it was possible to see in which part of 
the prominence these great motions arose, and to follow the 
change of wave-length to its extremcst limit. 
• Hy the kindness of Dr. Balfour Stewart, I am able to 
{ e.\hibit to you some of the Kew sun-pictures which sbo* 
yoii how these spectroscopic changes arc sometimes con- 
nected with telescopic ones. 

On the 2ist April there was a spot vciy near thelin* 
xvhich I was enabled to observe continuously for some 
time. At 7.30 A.M. there was a prominence visible in the 
(leld of view, in which tremendous action was evidently 



FIRST RliSULTS OF THE NEW METHOD. 



^ 



going on. for the C, D, and F lines were magnificently cha 
bright in the ordinary spectrum itself, and as the spot- 
spectrum was also visible, it was seen that the prominence 
was in advance of the spot. The injection into the a ^ 
chromosphere surpassed anything I had seen before, for ""7 
there was a magnesium cloud quite separated from the limb, ahe 
and high up in the prominence itself, ''" 

By 8.30 the action had quieted down, but at g.30 another 
throb was observed, and the new prominence was moving 




away with tremendous velocity. While this was going on, 
the hydrogen lines suddenly became bright on the other 
side (the earth's side) of the spot, and widened out con- 
siderably — indeed to such an extent that I attributed 
their action to a cyclone, although, as you know, this was 
a doubtful case. 

Now, what said the photographic record .' The sun was 
photographed at loh. 55m. A.M., and I hope you will be 
able to see on the screen how the suns surface was disturbed 
near the spot- A subsequent photograph at 4h. im. p.m. 
on the same day shows the limb to be actually broken 
io that particular place : the photosphere seems to have 



thephele- 
grafh at 
Iheplace 
■tvhrn Iht 




SOLAR PHYSICS. 

been absolutely torn away behind the spot, exactly where 
the spectroscope had afforded me possible evidence of a 
cyclone ! 

In connection with the last branches of the reseaidi I 
have brought to your notice, I may remark that' we have 
two very carefully prepared recent maps of the solar 
spectrum, one by Kirchhoff, the other by Angstrom, made 
a few years apart and at different epochs with regard to 
the sun-spot period. If you look at these maps, you will 
see a vast difference in the relative thicknesses of the C 
and F lines, and great differences in the relative darknes 



FiCr lot — CompuisoD of ^ and ^dJAomt lioet, u mapped by Kirchhotf Md Ki^v im 

and position of the lines ; and if I had time, I could shoi 

you that we now may be supplied with a barometer, so to 

speak, to measure the varying pressures in the solar and 

. stellar chromospheres ; for, depend upon it, every star ha 

has had, or will have, a chromosphere, and there are no 

such things as " worlds without hydrt^en," any more thM 

Atflica- there are stars without photospheres. I suggested in iMA 

''ratM '*/ ^^^ possibly a spectroscopic examination of the sun'j 

loiaritmly limb might teach us somewhat of the outburst of the star 

'" 'uar,"^ '" Corona, and already we see that all that is nccessar)' 

to get just such an outburst in our own sun is to increa$c 



FIRST RESULTS OF THE NEW METHOD. 239 



the power of his convection currents, which we know to chap.xiv. 
be ever at work. Here, then, is one cataclysm the less in 
astronomy — one less '* World on Fire," and possibly also 
a bright light thrown on the past history of our own 
planet. 

I might show you further that we now are beginning to 
have a better hold on the strange phenomena presented by 
variable stars, and that an application of the facts I have AppUca- 
brought to your notice this evening, taken in connection ^^abie 
with the various types of stars^ which have been indicated stars. 
by Father Secchi with admirable philosophy, opens out 
generalizations of the highest interest and importance ; 
and that, having at length fairly grappled with some of 
the phenomena of the nearest star, we may soon hope for 
more certain knowledge of the distant ones. 

At present, however, we may well leave speculation for 
those who prefer it to acquiring facts ; let us rather, em- 
boldened by the w^ork whidi this new method of research 
has enabled us to accomplish in this country, under the 
worst atmospheric conditions, in seven short months, go on 
quietly deciphering one by one the letters of this strange 
hieroglyphic language which the spectroscope has revealed 
to us — a language written in fire on that grand orb which 
to us earth-dwellers is the fountain of light and heat, and 
even of life itself. 

* When this was written I did not know that Mr. Rutherford was 
the first to suggest this field of research. 



THE AMERICAN ECLIPSE, 1 



I.— First Impressioms. 



CHAP. XV. If our American cousins in greneial heaitate to \ 
little island, lest, as some of them have put it, the] 
fall over the edge ; those more astronomically incliid 
vety fairly decline, on the ground that it is a sp< ' 
the sun steadily refuses to be eclipsed. This i 
tantalizing, because the Americans have jiist ob 
third eclipse this century, and already I have been \ 
to another, which will be visible in Colorado. Tot 
journey from Boston (I suppose I am r^ht in ra 
from Boston .') on July 29, 1878, 

Thanks to the accounts in StUiman's Journal \ 
Philosophical Magazine, and to the kindness of ] 
Winlock and Morton, who have sent me some \ 
photographs, I have a sufficient idea of the obe 
this third eclipse, which happened on the 7th ■'**' ^ 
last, to make me anxious to know very much |h 
them — an idea sufficient also, I think, to jn 
remarks on what we already know. 





THE PHOTOGRAPHIC CORONA 1869 
AT SHEXBYVIILE HENTUCKT 



THE AMERICAN ECLIPSE, 1869. 241 



because at such times the dark body of the moon, far out- chap. xv. 
side our atmosphere, cuts off the sun's light from it, 
and round the place occupied by the moon and moon- 
edipsed sun there is therefore none of the glare which we 
usually see — a glare caused by the reflection of the sun's 
light by our atmosphere. If, then, there were anything 
surrounding the sun ordinarily hidden from us by this glare, 
we ought to see it during eclipses. 

In point of fact, strange things are seen. There is a 
strange halo of pearly light visible, called the corona, and 
there are strange red things, which have been called red 
flames or red prominences, visible nearer the edge of the 
moon — or of the sun which lies behind it. 

Now, although we might, as I have pointed out, have 
:hese things revealed to us during eclipses if they belonged 
to the sun, it does not follow that they belong to the 
>un because we see them. Halley, a century and a half Haiifymp' 
ago, was, I believe, the first person to insist that they were ^'J^^^jV 
appearances due to the moon's atmosphere, and it is only jinmes 
writhin the last decade that modern science has shown to ^''^'"W^ 

tht mocn. 

everybody's satisfaction — by photographing them, and show- 
ily that they were eclipsed as the sun was eclipsed, and 
did not travel with the moon— that the red prominences 
really do belong to the sun. 

The evidence with regard to the corona was not quite 
so clear, but I do not think I shall be contradicted when I 
say, that prior to the Indian eclipse last year the general 
notion was that the corona^ was nothing more nor less than 
the atmosphere of the sun, and that the prominences were 
things floating in that atmosphere. 

While astronomers had thus been slowly feeling their 
^ay, the labours of VVolIaston, Fraunhofer, Herschel, Fox 
Talbot, Wheatstone, Kirchhoff, and Bunsen were providing 
^hem with an instrument of tremendous power, which was 
^^ expand their knowledge with a suddenness almost 
^^^rtling, and give them previously undreamt-of powers 

^ See note on page 243. 

K 



SOf.rIK /•HYSiCS. ' 

of research. I allude to the spectroscope, which was first ' 
successfully used to examine the red flames during the 
eclipse of last year. That the red flames were composed 'i 
of hydrogen, and that the spectroscope enabled us to study j 
them day by day, were facts acquired to science indepeO' | 
dcntly by two observers many thousand miles apart. 



t 




The red flames were " settled," then, to a certain extcat; 
but what about the corona ? 

After I had been at work for some time on ibe of" 
method of observing the red flames, and after Dr. Frwt- 
land and myself had very carefully studied the hydrt^ 
spectrum under previously untried conditions, wc camcit: 



THE AMERICAN ECLIPSE, 1869. 243 

the conclusion that the spectroscopic evidence brought chap. xv. 
forward, both in the observatory and in the laboratory, was 
against any such extensive atmosphere as the corona had and 
been imagined to indicate; and we communicated our Lockyefs 
conclusions to the Royal Society. Since that time, I con- attd reser- 
fess, the conviction that the corona * is nothing else than «'«^»- 
an effect due to the passage of sunlight through our own 
atmosphere near the moon's place has been growing stronger 
and stroi^r; but there was always this consideration to be 
b0rm$ in mind, namely, that as the spectroscopic evidetice 
di^mds mainly upon the brilliancy of tlie lines, that evidence 
wnr m a certain sense negative only, as the glare might defeat 
tim tfeetroseope with an uneclipsed sun in the coronal regions, 
Mlwnr Uu temperature and pressure are lower than in the 
fwi'Jbmu region. 

The great point to be settled then, in America, was, Point to be 
Wliat is the corona ? and there were many less ones. For ^^ ' 
fmtft~^, by sweeping round the sun with the spectroscope, 
bodi before and after the eclipse, and observing the pro- 
mlllT-^** with the telescope merely during the eclipse, we 
irfw—M get a sort of key to the strange cypher-band called 
the spectrum, which might prove of inestimable value, not 
oaljr in the future, but in a proper understanding of all 
tlie tdescopic observations of the past. We should, in fact, 
be tiios able to translate the language of the spectroscope. 
AgeiOt by observing the spectrum of the same prominence 
boCh before and during, or during and after the eclipse, the 
effect of the glare on the visibility of the lines could be 
determined— but I confess I should not like to be the observer 
charged with such a task. 

What, then, is the evidence furnished by the American The corona 
observers on the nature of the corona ? It is bi::arre and ^t^^^ou'r 
puzzling to the last degree ! The most definite statement aurora. 
on the subject is, that it is nothing more nor less than a 

* This word was applied when it was written to the coronas one or two 
solar diameters high, with rays some degrees long ; and not, as some 
have asserted, to the coronas which are seen before and after totality. 



SOLAR PHYSICS. 

. permanent solar aurora ! the announcement being founded 
on the fact, that three bright lines remained visible after 
the image of a prominence had been moved away from the 
slit, and that one {if not all) of these lines is coincident 
with a line (or lines) noticed in the spectrum of t)\c aurora 
borealis by Professor Winlock. 

Now it so happens that among the lines which I have 
observed up to the present time — some forty in number— 
this line is among those which I have most frequently 




iK9.i>tiuiH4)4 



fact, the first iron line which make 
its appearance in the part of the spectrum I generally 
study when the iron vapour is thrown into the chronn*- 
sphere. Hence I think that I should always see it if ihf 
corona were a permanent solar aurora, and gave out Oiii 
as its brightest line; and on this ground alone I shouW 
hesitate to regard the question as settled, were the ne* 



THE AMERICAN ECLIPSE, 1869. 245 

hypothesis less startling than it is. The position of the chap. xv. 
line is approximately shown in the woodcut (Fig. 90) near 
E, together with the other lines more frequently seen. 

It is only fair, however, to Professor Young, to whom 
is due this important observation, to add that Professor 

Harkness also declares for one bright line in the spectrum ^ ^^„. 

of the corona, but at the same time he, Professor Pickering, tinuous 

and indeed others, state its spectrum to be also continuous, g^J^^ 

a remark hard to understand unless we suppose the slit to ^^ corona. 




Fic. aof.*— General view of the protuberances, American Eclipse, August 7th, 1869. 

have been wide, and the light faint ; in either of which 
cases final conclusions can hardly be drawn either way. 

So much, then, for the spectroscopic evidence with which 
we are at present acquainted on the most important point. 
The results of the other attacks on the same point are i^ckermg's 
equally curious and perplexing. Formerly, a favourite ^^^^^\ 
arg^ument has been that because the light of the corona vations. 
is polarized, therefore it is solar. The American observers 



246 SOLAR PHYSICS. 



CHAP. XV. state that the h'ght is not polarized^ — a conclusion, as M. 
Faye has well put it, ''very embarrassing for Science" 
Further, — stranger still if possible, — it is stated that another 
line of inquiry goes to show that, after all, Halley may be 
right, and that the corona may really be due to a lunar 
atmosphere ! 
Question of I think I have said enough to show that the question 
^^^sdtUdT^ of the corona is by no means settled, and that the new 
method has by no means superseded the necessity of care- 
fully studying eclipses ; in fact, their observation has become 
of much greater importance than before ; and I earnestly 
hope that all future eclipses in the civilized area in the 
old world will be observed with as great earnestness as the 
last one was in the new. Certainly, never before was an 
eclipsed sun so thoroughly tortured with all the instruments 
of Science. Several hundred photographs were taken, unth 
a perfection of finish which may be gathered from the 
accompanying reproduction of one of them in Fig. 103. 

The Government, the railway and other companies, 
and private persons threw themselves into the work with 
marvellous earnestness and skill ; and the result was that 
the line of totality was almost one continuous observator}% 
from the Pacific to the Atlantic. We read in SiUimans 
Journal^ '* There seems to have been scarcely a town of any 
considerable magnitude along the entire line, which was not 
garrisoned by observers, having some special astronomical 
problem in view." This was as it should have been, and 
the American Government and men of science must be 
congratulated on the noble example they have shown to 
us, and the food for future thought and work they have 
accumulated.^ 

^ See a paper by Professor Pickering, Journal of thi FrmikUM 
Institute y December 1869, p. 373. 

' Since writing the above, I find the following independent tesdmoBjr 
in favour of Dr. Frankland's and my own notion of the corona in tlie 
Astronomische Nachrichteny from the pen of Dr. Gould. He says .^ 
" Its form varied continually, and I obtained drawings for three epodn 
at intervals of one minute. It was very irregular in form, and in no 



THE AMERICAN ECLIFSE, 1869. 



247 



11. My Communication to the Royal Society.^ 



CI! \\\ XV. 



b: duci- 
dateJ. 



By the kindness of Professors Winlock, Morton, and 
Newton, I have been favoured with photographs, and as yet 
unpublished accounts, of the results of the recent total 
eclipse of the sun observed in America. I am anxious, 
therefore, to take the opportunity afforded by the subject 
being under di.scussion, to lay a few remarks thus early 
before the Royal Society. 

The points which I hoped might be more especially Points to 
elucidated by this eclipse were as follows: — 

1. Is it possible to differentiate between the chromosphere 
and the corona } 

2. What is the real photographic evidence of the struc- 
ture of the base of the chromosphere in reference to Mr. 
W. De La Rue's enlarged photographs of the eclipse 
of i86o.J^ 

3. What is the amount of the obliterating effect of the 
illumination of our atmosphere on the spectrum of the 
chromosphere ? 

4. Is there any cooler hydrogen above the prominences ? 

5. Can the spectroscope settle the nature of the corona 
during eclipses } 

With regard to i, the evidence is conclusive. The chro- 
mosphere, including a " radiance," as it has been termed by 
Dr. Gould (the edge of the radiance as photographed being 
in places strangely like the edge of the chromosphere 
viewed with the open slit), is not to be confounded with 
the corona. 

On this subject, in a letter to Professor Morton, Dr. B. 
A. Gould writes : — " An examination of the beautiful 



Point I. 



apparent relation with the protuberances on the sun, or the position of 
the moon. Indeed, there were many phenomena which would almost 
lead to the belief that it was an atmospheric rather than a cosmical 
phenomenon. One of the beams was at least 30' long." (1869.) 

1 Received December 7, 1869 ; printed in Proc. R, S. vol. xviii. 
p. 179- 



SOLAR PHYSICS. 

photographs made at Burlington and Ottutnwa by the 
sections of your party in charge of Professors Mayer and 
Haines, and a comparison of them with my sketches of 
the corona, have led me to the conviction that the radiance 
around the moon in thi; pictures made during totality is R'.'t 




the corona at all, but is actually the image of what Lockycr 
has called the chromosphere. 

" This interesting fact is indicated by many diffcrcni 
considerations. The directions of maximum radiance io 
not coincide with those of the great beams of tlie corona; 
they remain constant, while the latter were variable. There 
is a diameter approximately corresponding to the solai 




THE AMERICAN ECUFSE, 1869. 



349 



axis, near the extremities of which the radiance upon the chap. xv. 
photographs is a minimum, whereas the coronal beams in ~ ~, 
these directions were especiaHy marked during a great relaiian 
part of the total obscuration. The coronal beams stood l>f^<"™'^' 
1 no apparent relation to the protuberances, whereas the bams and 




I aureole seen upon the photographs is most marked in their 
] immediate vicinity ; indeed the great protuberance, at 230° 
I to 245°, seems to have formed a southern limit to the 
I radiance on the western side, while a sharp northern limit 
I is seen on all the photographs at about 350°, the intermediate 
I arc being thickly studded with protuberances which the 
I inoon displayed at the close of totality. The exquisite 



2 so SOLAR PHYSICS. 



CHAP. XV. masses of flocculent light on the following limbs are upon 
the two sides of that curious prominence at 93% which at 
first resembled an ear of corn, as you have said, but which, 
in the later pictures after it had been more occulted, and 
its southern branch thus rendered more conspicuous, was 
like a pair of antelope's horns, to which some observers 
compare it. Whatever of this aureole is shown upon the 
photographs was occulted or displayed by the lunar motion, 
precisely as the protuberances were. The variations in the 
form of the corona, on the other hand, did not seem to be 
dependent in any degree upon the moon's motion. The 
singular and elegant structural indication in the special 
aggregations of light on the eastern side may be of high 
value in guiding to a further knowledge of the chromosphere. 
They are manifest in all the photographs by your parties 
which I have seen, but are especially marked in tliose of 
shortest exposure, such as the first one at Ottumwa. In 
some of the later views they may be detected on the other 
side of the sun, though less distinct ; but the very irr^ular 
and jagged outline of the chromosphere, as described by 
Janssen and Lockyer, is exhibited in perfection." 

Poiuf 2. The second point is afso referred to in the same letter. 
I think the American photographs afford evidence that 
certain appearances in parts of Mr. De La Rue's photo- 
graphs, which represent the chromosphere as billowy on 
its under side, are really due to some action either of the 
moon's surface or of a possible rare lunar atmosphere ; so 
that it is not desirable to confound these effects n-ith 
others that might be due to a possible suspension of the 
chromosphere in a transparent atmosphere, if only a sictm 
of the chromosphere were photographed. 
Or, Dr. Gould writes : — " You will observe that some of the 

^^^^ brighter, petal-like flocculi of light have produced apparent 
indentations in the moon's limb at their base, like those at 
the bases of the protuberances. These indentations are 
evidently due to specular reflection from the moon's surface. 
as I stated to the American Association at Salem last 



THE AMERICAN ECU PS E, 1869. 251 

month. Had any doubt existed in my mind previously, chap. xv. 
it would have been removed by an inspection of the 
photographs." 

Where the chromosphere is so uniform a light that the 
actinic effect on the plate is pretty nearly equal, the base 
of the chromosphere is absolutely continuous in the 
American photographs ; but in the case of some of the 
lai^er prominences, notably those at + 146 (Young) and 
— 130 (Young), there are strong apparent indents on the 
moon*s limb. 

I next come to the obliterating effect of the illumination p<nm j. 
of our atmosphere on the spectrum of the chromosphere. 

This is considerable ; in fact the evidences of it are very 
much stronger than one could have wished, but hardly 
more decided than I had anticipated. Professor Winlock's 
evidence on this point, in a letter to myself, is as follows : — 
" I examined the principal protuberance before, during, and 
after totality. I saw three lines (C, near D and F) before 
and after totality, and eleven during totality ; eight were 
instantly extinguished on the first appearance of sunlight^ 

This effect was observed with two flint prisms and seven 
inches' aperture. Professor Young, with five prisms of 45° 
and four inches aperture, found the same result in the part 
of the spectrum he was examining at the end of the totality. 

He writes: "I had just completed the measurements Professor 
of 2602, when the totality ended. This line disappeared *^"^^ 
instantly y but 2796 [the hydrogen line near G] was nearly 
a minute in resuming its usual faintness." These observa- 
tions I consider among the most important ones made 
during the eclipse ; for they show most unmistakably that, 
as I have already reported to the Secretary of the Govern- 
ment-Grant Committee, the new method, to be employed 
under the best conditions, must be used with large apertures 
and large dispersion. 

On the fourth point the evidence is but negative only, Pvini 4. 
and therefore in favour of the view I have some time ago 
communicated to the Royal Society. 



252 SOLAR PHYSICS. 



CHAP. XV. We next come to the question of the corona, — a question 
which has been made more difficult than ever (in appear- 
ance only, I think) by the American observations. 

I propose to discuss only the spectroscopic observations 
of Professors Young^ and Pickering in connection with Dr. 
Gould's before-quoted remarks. 

Professor Pickering, with an ordinary chemical spectro- 
scope, merely directed to the sun*s place during totalit}', 
obtained the combined spectrum of the protuberances and 
corona. He saw a continuous spectrum with two or three 
bright lines, one " near E," and a second " near C." 

Professor Young, who used a spectroscope specially 
adapted for the work, in which only one part of the pro- 
minence at -H 146'' was being examined, saw C, near D, 
a line at 1250 + 20, and another at 1350 ±20 of Kirch- 
hofTs scale. The rest of the observations I give in his 
own words : — 
Extension "Then came the 1474 K line, which was very bright, 
thepeen ^^ough by no means equal to C and Dj ; but attention was 
coronal immediately arrested by the fact that, unlike them, it 
^the whole extended clean across the spectrum ; and on moving the 
width of slit away from the protuberances, it persisted, while D,, 
M^' visibly in the edge of the field, disappeared. Thus it was 
evident that this line * belonged not to the spectrum of the 
protuberance, but to that of the corona. My impression, 
but I do not feel at all sure of it, is that the two faint lines 
between it and Ds behaved in the same manner, and are 
also corona lines.* 

^ Professor Young's observations will be given in exiensa in the 
Appendix. 

^ ** On two or three occasions previously I had been very mack 
surprised at not being able to detect this line in the spectnun of 
unusually bright prominences. On the other hand, I once found it 
very easy to see at a place on the sun's limb where the other chrano- 
spherc lines, usually far more brilliant, were almost invisible." 

s " A careful examination of the photographs, especially Na 3 of 
the Burlington totality pictures, somewhat diminishes my confideDce 
in the conclusion of the text as to the nature of these three lines 
(1250, 1350, and 1474). They certainly do not belong to the spcctrnm 



THE AMERICAN ECLIPSE, 1869. 253 

" I am confirmed in this opinion by Professor Pickering's chaf. xv. 
observation. He used a single-prism spectroscope, with 
the slit of the collimator simply directed to the sun, and /^2^f^^ 
having no lens in front of it. With this arrangement he obsenHt- 
saw only three or four bright lines, the brightest near E ^^' 
(1474). Now this is exactly what ought to occur if that 
line really belongs to the corona, which, from its great 
extent, furnished to his instrument a far greater quantity 
of light than the prominences. 

"By this time the moon had advanced so far that it 
became necessary to shift the slit to the great prominence 
on the opposite side of the sun. While my assistant was 
doing this, I suppose I must, in the excitement of the 
moment, have run my eye-piece over the region of the 
magnesium lines {b), and thrown them out of the field 
before he had brought anything upon the slit. At any 
rate I saw nothing of these lines, which were evident enough 
to several other observers, and can think of no other way to 
account for their having escaped me. The F line in the Appear- 
spectrum of the great protuberance was absolutely glorious, ome of the 
broad at the base and tapering upwards, crookedly, as 
Lockyer has before often observed. Next appeared a new 
line, about as bright as 1474 at 2602 4- 2 of Kirchhoffs 

of the most brilliant portion of the prominences ; but around the pro- 
minences of the eastern limb, on which the slit of the spectroscope was 
directed during the first half of the totality, the photograph shows a 
pretty extensive and well-defined nebulosity, evidently distinct from, 
though associated with, the brilliant nuclei. Now it is possible that 
these lines may belong to this nebulosity, and not to the corona proper ; 
for I cannot recall with certainty whether 1474 retained its brilliance 
at any considerable distance from the prominences, or only in their 
immediate neighbourhood. My strong impression, however, is that 
the former was the case, and that the text is correct. I may as well 
confess that my uncertain memory here is due to the fact that just at 
this time, while my assistant was handing me the lantern with which 
to read the micrometer-head, I looked over my shoulder for an 
instant, and beheld the most beautiful and impressive spectacle upon 
which my eyes have ever rested. It could not have been for five 
seconds : but the effect was so overwhelming as to drive away all 
certain recollection of what I had just seen. What I have recorded 
I recall from my notes taken down by my assistant." 



254 



SOLAR PHYSICS. 



ciiAP. XV. scale. Its position was carefully determined by micro- 
metrical reference to the next line, 2796 K (hydrogen 7), 
which was very bright ; h was also seen, very clear, but 
hardly brilliant. In all, I saw nine bright lines. 

" A faint continuous spectrum, without any traces of 
dark lines in it, was also visible, evidently due to the 
corona. Its light, tested by a tourmaline applied next to 
the eye, proved to be very strongly polarized in a plane 
passing through the centre of the sun. I am not sure, 
however, but that this polarization, as suggested by 
Professor Pickering, may have been produced by the 
successive refractions through the prisms. This explana- 
tion at once removes the difficulty otherwise arising from 
the absence of dark lines." 



The light 
of the 
corona 

polarized. 



I have first to do with the continuous spectrum, deduced 
from Professor Pickering's observations. 

I think in such a method of observation, even if the 
corona were terrestrial and gave a dark line spectrum, the 
lines visible with such a dim light would in great part be 
obliterated by the corresponding bright lines given out by 
the long arc of chromosphere visible, to say nothing of the 
prominences, in which it would be strange if C, D, E, *, F, 
and many other lines were not reversed. This suggestion, 
I think, is strengthened by the statement that two bright 
lines were seen " near C* and " near E ;" should we not 
rather read (for the " near " shows that we are only dealii^ 
with approximations) C and F, which is exactly what we 
might expect ? 

But even this is not all that may be hazarded on the 
subject of the continuous spectrum, which was also seen 
by Professor Young under different conditions. 

Assuming the corona^ to be an atmospheric effect mcrdy. 
as I have before asserted it to be, it seems to me that its 
spectrum should be continuous, or nearly so ; for is it not 
as much due to the light of the prominences as to the lyrt 

* See note on page 243. 



THE AMERICAN ECLIPSE, 1869. 255 



of the photosphere, which, it may be said roughly, are chap. xv. 
complementary to each other ? 

With regard to the aurora theory, I gather from Professor 
Young's note that, if not already withdrawn, he is anxious 
to wait till the next eclipse for further facts. I consider 
that the fact that I often see the line at 1474, and often do 
not, is fatal to it, as it should be constantly visible on the 
proposed hypothesis. The obserx-ation of iron-vapour, as 
I hold it to be at this elevation, is of extreme value, coupled 
with its simple spectrum, seen during an eclipse, as it 
entirely confirms my observations made at a lower level 
in the case, not only of iron but of magnesium. 

[The following communication from Professor Young, 
printed in Nature, March 24, 1870, commenting upon 
some of the views advanced above, is here given : — 

" It is not impossible that the so-called corona may be complex. 
Some portion of its radiance may, perhaps, originate in our own 
atmosphere, although I do not yet find myself able to accord with 
the conclusions of Dr. Gould and Mr. Lockyer in this respect, and 
am strongly disposed to believe that the whole phenomenon is purely 
solar. 

"This much appears certain, however, that there exists outside 
of the chromosphere properly so called (;>., the envelope of rea 
hydrogen), and as distinct from /"/ as it is from the photosphere, 
an immense atmosphere of self-luminous substance, extending to 
a distance of from 5' to 8' from the sun's surface, and probably 
much further in places — phosphorescent dust or fog in a glowing gas. 

'* In support of this idea I adduce the photograph of Mr. Whipple, 
taken at Shelbyville, Ky., with an exposure of 40*. On this, the 
photolytic corona (if I may use the expression to distinguish it from 
the visible corona, whose points of maximum brilliance were, accord- 
ing to Dr. Gould, entirely different) reaches a height of 6'. Professor 
Harkness observed the 1474 line in the spectrum of the corona at a 
distance of nearly 5' from the sun's limb, and not near to any promi- 
nence. I do not know the precise elevation at which I saw it, but it 
was not less than 3' or 4'. 

'* Indirectly, also, the idea is confirmed by the spectroscopic obser- 
vation of Professor Pickering, who used a single prism instrument, 
with the slit simply directed towards the sun, not attached to a 
telescope. He saw only three or four lines, the brightest in the 
green near E. Now, since this line, when observed by throwing 
a large image of the sun on the slit, is very faint as compared 
with C, D3, and F*, its intensity, as seen by him, can only be 



256 SOLAR PHySICS. 



CHAP. XV. accounted for by supposing that the luminous area from which it 

was derived far exceeded that of the chromosphere and prominences. 

" I have noticed also that some of the ooservers of the Indian 
eclipse (Rayet and Pogson) speak of the intensity of the green line. 
Did they observe in the same manner as Professor Pickering? 

" I need hardly add that Professor Pickering's observation of the 
non-polarization of the corona concurs with what has been said. 

'^ As to the faint continuous spectrum, I am sure that the reported 
absence of dark lines, was not the result of insufficient observation. 

'* I could not have failed to see D, Eyb, 1961, F and G, had they 
existed ; for in a spectrum of similar brightness fonned by a light 
from a cloud, not only these but many other lines are visible in mj 
instrument. Now, the absence of some of these might, perhaps, be 
accounted for on the ingenious hypothesis proposed by Mr. Lockyer; 
but this would not apply to D, E, or G. [Why not ? — J. N. L.] 

'* But if we admit the existence of faintly luminous solid or foggy 
matter near the sun, either meteoric or arising from the cloudy con- 
densation of a non-permanent gas, the whole is at once easy of com- 
prehension." 

The auroral theory of the corona is thus referred to : — 

" The objection pressed by Mr. Lockyer that the bright line 1474 
is only occasionally visible, is, I think, unfounded. At any rate I have 
never failed to see it myself when looked for, and very seldom to 
make it visible to others when I have wished to exhibit to them. It 
is faint, and, like a difficult microscopic object, requires management 
to bring it out with five prisms ; but by placing the slit tangential to 
the sun's disc, and giving the instrument a slight jar, it is seen to 
flash out as the limb passes off the slit. It is worth noting, too, 
that it is often especially plain at portions of the limb where the 
chromosphere is unusually shallow and faint. 

*'But while I think it probable that this line coincides with the 
aurora line reported by Professor Winlock at 1550 of Mr. Huggins* 
scale, I am by no means sure of it. I understand its assigned position 
rested upon a single observation with a chemical spectroscope, and 
the probable error of such a determination cannot well be less than 
ten divisions of KirchhofTs scale. I have naturally made many 
attempts to determine its position for myself, but have never seen it 
except thrice, and then not long enough at a time to complete a 
measurement. I am only sure that its position lies between 1460 and 
1490 of Kirchhoff. 

'* For this reason, although I do not at all abandon the hypothesis, 
which appears to have other elements of probability in the general 
appearance of the corona, the necessity of mtense electrical disturb- 
ances in the solar atmosphere as the result of the power^l vertical 
currents known to exist there, as well as the curious responsiveness d 
our terrestrial magnets to solar storms ; yet I do not feel in a position 
to urge it strongly, but rather await developments. 

'* As to the substance which causes this line, I observe that Father 
Secchi, in a recent communication to the French Academy, is dis- 
posed to think it hydrogen ; while Mr. Lockyer still belie\*es it to be 



THE AMERICAN ECLIPSE, 1869. 



257 



iron. I am in hopes that experiments now in progress may throw 
some light on the subject. 

" May I suggest, in closing this long communication, that it seems to 
me that valuable observations might be made at the Eclipse of next 
December, by fitting up telescopes with a ground glass sliding screen, 
upon which an image of the corona two or three inches in diameter 
should be thrown ; the ground glass having the roughened side next 
the observer, so that he could sketch upon it with a lead pencil the 
outlines of the image, the glass being made long enough to allow of 
several such sketches. 

** The comparison of a series of such outlines would decide the 
question of changes in the coronal streamers, as the sketches, being 
simple tracings, could not but be accurate in their indications of 
position."] 



CHAP. XV. 

Professor 

Young s 

leiter. 



THE MEDITERRANEAN ECLIPSE, i8 



I.— A Letter from Venice.* 



CHAP. XV J. 



Professor 
Peircis 
letter. 



Cloud in Sicily, cloud in Spain, cloud in Africa. 2 
first sight might seem to be the only result of 
observations made on the eclipsed sun of 1870; si 
reception given by Nature to those who wooed hei 



^ I should be wanting in gratitude if I did not record one 
in connection with this expedition. Owing to those whos 
was to bring the requirements of science before the Gov 
appealing to the wrong department, and other causes, funds ti 
observations were in the first instance refused. At this juncti 
fessor Peirce, the distinguished director of the expedition i 
in the most liberal manner by the American Government, 
in this country, and hearing of the position of affairs, at \ 
me the honour to send me the following letter : — 

" FENTON'fe H 

" My dear Sir, — I have been directed by the Govemmex: 
United States to have the best possible observations made (tf 
eclipse of next December. If I could aid the cause of astroi 
assisting the observers of England in their investigations 
phenomenon, I should be greatly pleased. I take the liberty t 
to invite your attendance, and also that of other eminent pi 
of England, with either of the panics of my expedition, one fl 
will go to Spain and the other to Sicily. 

" Yours very respectfully and faithfully, 

"Benjamin Pi 

"J. Norman Lockver, Esq. F.R.S." 



I at once accepted this crenerous offer. 



THE MEDITERRANEAN ECLIPSE, 1870. 259 

had never been wooed before, who approached her full of chap.xvi. 
the rarest gifts which Science has placed at man's disposal. 
But, after all, has the oracle been silent ? I think not 
Dare we, however, say that the great problem of the 
Corona, that one among the many still outstanding diffi- 
culties which the eclipse was invoked to settle, is settled ? 
This, perhaps, would be saying too much, but still, I think, 
a step in advance has been made. The oracle has spoken 
darkly perhaps, but it has spoken. 

upon to state the requirements of science to the Chancellor of the 
Exchequer Mr. Lowe, and Mr. Stansfeld, who thus heard of the matter 
for the first time. Nothing could exceed the anxiety at once shown 
to aid the work ; and the subsequent supply of ships, money, and 
general aid left nothing to be desired. 

The expedition which eventually left England was made up as 
follows : — 

A.— Spanish and Algerian Party. 

/. Cadiz detachtnent. In charge — The Rev, S» J, Perry, 

spectroscope — The Rev. S. J. Perry and assistant (Mr. Hostage), 
Mr. Abbay. 

Polariscope — Mr. Moulton, Mr. Hudson, Mr. Fison. 

Sketches of Corona — Mr. Naftel, Mr. Smyth, Mr. Penrose, Mr. 
Collins. Time and General Observations — Captain Toynbee. 

//. Gibraltar detachment. In charge— Capt. Parsons, 

spectroscope — Mr. Carpmael, Mr. Gordon. 

Polariscope — Mr. Lewis, Mr. Ladd, Mr. Baynes. 

Photography — Mr. Buckingham and assistant. 

Sketches of Corona — Mr. Hunter, Mr. Anson, Mr. Harrison. 

Saturn in the Corona — Mr. Talmage, Mr. Maclear. 

///. Oran detachment. In charge — Mr, Muggins, 

Mr. Huggins, Admiral Ommanney, Lieutenant M. F. Ommanney, 
Professor Tyndall, Rev. F. Howlett, Mr. Carpenter, Mr. Crookes, Cap- 
tain Noble, Dr. Gladstone. 

B.— Sicilian Party. 
In charge — Mr, Lockyer, 

Spectroscope — Mr. Lockyer and assistant (Mrs. Lockyer), Professor 
^oscoe and assistant (Mr. Bowen), Mr. Seabrooke, Mr. Pedler, Mr. 
^Urton. 

,^^olariscope — Mr. Ranyard, Mr. Griflfith, Mr. Clifford, Mr. Harris, 
*^X>fessor Adams. 

'Sketches of Corona — Mr. Brett, Mr. Darwin. 

Photography— }Ax, Brothers, Herr Vogel, Mr. Fryer. 
• '^ime and ueneral Observations — Mr. Vig^oles, sen., Mr. Vignoles, 
i^ii._(7^/«iV<i/ Intensity— :Fro(essoY Thorpe. 



26o SOLAR PHYSICS. 

cuAP.xvr. Let me endeavour to put the question as it stood a feu* 
weeks ago as briefly as possible. 

Beginning the story some few years back, we find the 
corona, a halo of white light round the moon, with a 
height sometimes represented as equal to the moon's 
diameter, sometimes more, sometimes less, with a border 
a discrdtiony — so much did the drawings vary — regarded as 
the solar atmosphere. 

Some thought the red prominences to be mountains, 
other observers called them clouds. 

The The polariscope was brought up with a view of d^ 

^usv(L^^ termining whether the corona shone by reflected light or 
not. The result of this new method of observation was 
doubtful. 

In the Indian eclipse of 1868 M. Janssen, by means of 
the spectroscope, still another aid, determined that the 
prominences were masses of hydrogen gas, but there was 
no final word about the corona. Major Tennant observed 
that its spectrum was continuous. Later in the same year 
Dr. Frankland and myself approximately determined the 
pressure of the prominence gases by means of the ne» 
method and laboratory experiments, and at once stated 
our conviction that the extensive corona which had been 
depicted and represented by Kirchhoff" and others to be 
the solar atmosphere must be something else. This was 
our idea. I cannot quote our words, for I am \iTiting 
in Venice and have no copies of our paper with me. 

7 he In the American Eclipse of 1869 the problem was 

^ec/lpse!' advanced considerably, perhaps even more considerabix 
than we can yet form an idea of, writing as wc most 
still do doubtfully. I do not refer to the drawings, fo 
they varied considerably, but to the observation that the 
light of the outer corona, like that of the promincnoOi 
gave a bright-line spectrum. But as at least some of Ac 
observers gave positions doubtfully, "near C" and "netf 
E," I thought that the explanation was still possible wUdi 
regarded the corona as of terrestrial origin ; that is, whidi 



THE AfEDITERRANEAN ECLIPSE, 187a 261 



assumed it to be an appearance due to the presence of chap.xvi. 

light in our own atmosphere. The problem was one of 

such difficulty that there seemed a possibility that, by 

some unexplained cause, some of the solar light might 

be diffused and beat out of its course, and then, mixing 

up with the light of the chromosphere, give us a sort 

of continuous spectrum, with the hydrogen bright lines 

superposed upon it ; in other words, that as the eye 

perceives a bright, irregular region or glare around the 

uneclipsed sun, an effect due to our atmosphere, so also 

the eye might perceive a bright, irregular region or glare 

round the uneclipsed chromosphere during eclipses, due also 

to our atmosphere. 

One word here about the Chromosphere, the name I 
have given to the bright-line-giving region outside the 
photosphere. It has long been clear that the spectroscopic 
method of observing it when the sun is not echpscd is not 
totally effective ; that is to say, that we only see a per- 
centage of it — perhaps only a relatively small percentage — 
hut the glowing prominences, that is, those in which there 
is no evidence of the rapid motion of ejection from the 
sun, the ejection taking place at all angles from the line 
of sight, afford evidence that there is probably a layer 
of cooler hydrogen susceptible of being rendered visible 
*bove the ordinary level. Now as these prominences 
may be 5' high, it is not unreasonable to suppose that 
the chromosphere may even extend to that distance, or 
even a little beyond it.^ 

* Here is what I wrote on this point a year ago : — " I next come to q^^ 
^ obliterating effect of the illumination of our atmosphere en the atmosphrre 
•pcctmm of the chromosphere. This is considerable ; in fact )he partly 
^^'idences of it are very much stronger than one could have wished, obiUeraUs 
hot hardly more decided than I had anticipated. Professor \Vinlc)ck*s thfihronio- 
^cnce on this point, in a letter to myself, is as follows : — * I examined spha-t. 
^ principal protuberance before, during, and after totality. I saw 
^hree lines (C, near D, and F) before and after totality, and eleven 
<lttring totality ; eight were instantly extinguished on the first appear- 
^^ of sunlight ' This effect was observed with two flint prisms and 
7 inches' aperture. Professor Young, with five prisms of 45' and 4 
inches' aperture, found the same result in the part of the spcctnim he 



262 SOLAR PHYSICS, 

CHAP. XVI. Hence it was that in the Instructions to Observers,* 

drawn up by Professor Stokes and myself, and approved 

by the Organizing Committee for this 1870 Eclipse, it is 

stated that — 

Objects 0/ "The PRINCIPAL OBJECT to be obtained is to detcr- 

^^\hnof n^'"^ whether it is possible to differentiate the outer 

1870. layers of irregular outline and the streamers (of the 

corona) from a stratum, say some S' or 6' high, round the 

sun, which may possibly be the limit of the gaseous 

envelopes above the photosphere." 

The spectroscopic observers, therefore, were enjoined— 
{a) ** To determine the actual height of the chromosphere 
as seen with an eclipsed sun : that is, when the atmo- 
spheric illumination, the effect of which is doubtless only 
partially got rid of by the Janssen-Lockyer method, is 
removed. If the method were totally effective, the C line, 
the line of high temperature, should hardly increase in 
height ; but there can be little doubt that the method is 
not totally effective, so the increase in height should be 
carefully noted." 

{b) " To determine if there exists cooler hydrogen abow 
and around the vividly incandescent layers and pro- 



minences." 



And the polarizers 

" To examine a detached and selected part of the corona 
about 6' from the limb of the sun, and say about 8' in 
diameter." 

Having got so far, it may be here stated that of the 
three means of attack, namely, the spectroscope, the 

was examining at the end of the totality. He writes :— M hadjast 
completed the measurements of 2602, when the totality ended. Tfc 
line disappeared instantly^ but 2796 (the hydrogen line near G) w* 
nearly a minute in resuming its usual faintness.' These observaboH 
I consider among the most important ones made during the edipM: 
for they show most unmistakeably that, as I have already reported tt 
the Secretary of the Government-Grant Committee, the new method 
to be employed under the best conditions must be used with Urp 
apertures and large dispersion." — Proc. A*. »S*., 1870, p. 181. 
* These will be found in the Appendix. 



THE MEDITERRANEAN ECLIPSE, 1870. 



263 



polariscope and telescope, and naked-eye observations, 
the spectroscopic method, under certain circumstances, 
might have been by far the most doubtful, the polariscope 
method coming next. 

With regard to the spectroscopic observations, if we 
assume that no light whatever is received by and from 
our own atmosphere, the observations would be easily 
translated. A pure continuous spectrum would reveal to 
us solid or liquid matter in the circumsolar regions ; a 
spectrum continuous or not containing bright lines would 
give us gases or vapours ; the ordinary solar spectrum, 
with its dark lines, would indicate matter incapable of 
radiation itself, and therefore cool, reflecting to us ordi- 
nary sunlight. It is clear that the problem would be 
complicated if circumsolar matter both reflected sun-light 
and sent us its own ; and still more so if we allow that 
the coronal light may be partly contributed from reflec- 
tions and refractions in our own atmosphere. Then we 
have to consider whether the light thus contributed may 
possibly be due to the photosphere or to the prominences, 
and we are landed in a maze of difliculties which need not 
be discussed here. 

The system of sketching introduced for this eclipse is 
at once so simple and final that the only wonder is it has 
not been introduced before. The corona must be either 
solar, atmospheric, or subjective, that is, more or less built 
up in the observer's eye, this more or less depending 
cateris paribus upon the brilliancy of the undoubted solar 
portion. If at all stations, the stations being as wide apart 
as they have been this time, the drawings are similar, then 
the corona would be undoubtedly cosmical ; if dissimilar, 
then it would either be terrestrial or subjective : and this 
point could and would have been settled this time, if the 
weather had permitted, by arranging the observers inpairs^ 
— ^that is, dealing with two observers in each place instead 
of a single one, and so obtaining the eye-variation. 

This being premised, what is the result of the vcr>' few 



CHAP. XVI. 



Meaning 
of the spec- 
troscopic 
indica- 
tions. 



Use of 
sketches 
made at 
different 
places. 



264 SOLAR PHYSICS, 



cHAP.xvi. observations, comparatively speaking, which have been 
made ? Before I attempt to give any idea of my answ'cr 
to this question, it is only fair to myself to state that my 
only sources of information, up to the present time, have 
been conversations with some of the American members 
of the Sicilian expedition, a brief telegram from the mem- 
bers of the English party at Agosta, the Rev. S. J. Perry's 
communication to the Daily News of the 2nd instant, and 
an inspection of some drawings made by the officers of 
H.M. ships off Aci Reale. At Catania we saw a portion 
of the corona for i^ seconds through a cloud, and that 
was all ; and the day after the eclipse, before the more 
fortunate members of my party returned, it became my 
duty to proceed to Malta in H.M.S. Lord Warden to 
attend the court-martial on the officers and crew of the 
beautiful, but unfortunate Psychey in which we had been 
wrecked on the 15th ult., and the weather in the Medi- 
terranean has been so bad that it was impossible to leaw 
Malta in time to rejoin the expedition before they left 
for England. Of detailed information, therefore, I have 
none. 
What the In the first place, then, I submit that the fact that the 
'T'T''^^ ^"tT ^^^^"^ '^ ^ compound phenomenon comes out in an un- 
drawings, mistakablc way. We have first of all a ring some 5' or 
(y high round the moon, which almost all observers have 
seen alike ; and then we have light beyond which some 
observers have seen of one shape and some of another, 
now stellate with many rays, now stellate with few, now 
absolutely at rest, now revolving rapidly. 

This I think to be the key-note of all the observations 
with which I have become acquainted. I need scarcely 
say that it is exactly what had been predicted. 

First among the fortunate ones who observed the corooa 
with the telescope was Prof Watson, of Ann Arbor, who 
took up his station at Carlentini, and appears to have been 
the best favoured among the Sicih'an observers. From ht< 
account I gather that there was an almost perfect skil 



THE MEDITERRAXEAS ECUPSE. 1870. 265 



ccrtma. 



around the sun about 5' high, and that outside this shell chap.xvt. 

were less definite rays. WTiat he ^n'as particularly struck ~~~ T 

with was this, that, as seen in the telescope, the rayed apfta/ame 

portion was most developed over the prominences, and, as ^^j^ 

I gathered from him in one case, the rayed portion was 

absent as if a veil had been removed ; so that he, at all 

events, is strongly impressed with the idea that the shell 

represented a true solar appendage, and that the rayed 

structure was due to our own atmosphere. 

Next comes Mr. Brett, who, although he was not so 
fortunate, still was enabled to see and place on record 
5ome most interesting features, including the whole out- 
'ine of the corona and even some of the protuberances, 
tte also, as I am informed, saw the rayed portion of the 
corona most developed above the protuberances, the outline 
of the interior portion being visible, though not so strongly 
^a.rked as in the case of Prof. Watson's drawing, in con- 
sequence of less favourable atmospheric conditions. I am 
^^nkful to say that the weather at Syracuse enabled Mr. 
^^others to obtain some admirable photographs, which I 
ha.\re not yet seen. These are among the most important 
>^^3\ilts of the Expedition. 

^ext I must mention Prof Peirce, the head of one of 
^"^^ American parties, who observed two miles north of 
^^tania, at a private casino of the Marchese Sangiuliano. 
* l^clieve that he also saw the shell, but of this I am not 
^^solutely certain ; but he distinctly observed that the 
^'^ter corona over the prominences was rosy red, although 
"^ did not see the prominences himself A more beau- 
"*l proof of the terrestrial nature of this portion of the 
^^i"ona it would be difficult to imagine ; for, of course, at 
'*^^ sun, the hydrogen, which thus tinged it, is incapable of 
^^louring anything, as its own light is absorbed by the 
**"^nscendent brilliancy of the photosphere ; while nothing 
^*^uld be more natural than to suppose that the light, 
^"Jiich in its own atmosphere should strongly tinge anything 
^^dially illuminated, should be that of the prominences. 



266 



SOLAR PHYSICS. 



CllAP.XVI. 



Ail the 

draiuings 
made on 

bMrd the 
men-of- 
war at 

Aci ReaU 
were 

different. 



But the strongest proof of the variability of the outer 
portion and of the constancy of the inner portion is 
afforded by the observations made on board the small fleet 
attempting to save the PsycJu off Aci Reale, where the 
eclipse was observed in unclouded splendour. Here were 
the ironclads Lord Warden, Caledonia, and Royal Oak, 
and the tugs Weasel and Hearty, besides the Italian gun- 
boat Plebiscito, all within a stone's throw of each other. 
In all the drawings, and many have been received, we have 
a ring 5' or thereabouts, while the outer portion is as vari- 
able as may be. On the same deck, that namely of the 
flag-ship Lord Warden, two drawings were made, one by 
Capt Brandreth, and the other by Dr. Macdonald, F.R.S., 
in which the variation is so strong that one would feel 
inclined to acquit the atmosphere of any participation in 
the matter, and to relegate the whole outer corona to sub- 
jectivity alone, did not Mr. Brothers' admirable photographs 
show both phenomena, as I am told they do. Dr. Mac- 
donald saw eight rays arranged with perfect symmetr)'; 
Captain Brandreth saw only two elliptical hoops crossing 
each other at right angles. 

Captain Cochrane of the Caledonia, besides the ring, saw 
a complicated stellate figure, the rays of nearly equal 
length; while Mr. Dexter, at sea between Catania and 
Syracuse, saw, besides the ring, only one ray of inordinate 
length. 

So much for the drawings. I think that if the records 
of former eclipses be now examined, especially Mr. Car- 
rington's drawing of the eclipse of 185 1, and compared 
with the others taken at the same time, additional evidence 
will be gathered in favour of the compound nature of the 
corona, which, on the evidence now before me, I consider 
the great teaching of the present eclipse. Our experience 
in Sicily seems to be similar to that of the Spanish ob- 
servers, for Mr. Perry writes that " some obser\'ed tiro 
curved rays," while the rapid degradation of light occurred 
at one-fifth of a solar diameter, but, so far as I know, no 



THE MEDITERRANEAN ECLIPSE, 187a 



267 



one in Sicily was favoured with a view of the dark intervals 
which were observed in Spain. 

There is a strange and most interesting discordance 
between some of the spectroscopic observations made in 
Sidly and Spain. At Agosta, where the totality was well 
visible for ten seconds, Mr. Burton detected a green line 
near E, with a tangential slit (distance from moon not 
stated). This line, which was also seen by the Italian 
observers, is doubtless the one recorded last year by the 
American astronomers, but in Spain Mr. Perry states that 
bright lines at C near D, b (or E) and F were observed 8' 
away from the sun. At Syracuse Prof. Harkness, whose 
telescope was moved into the various positions by Captain 
Tupman, R.M.A., found the green line in all parts of the 
corona, so far as about 10' from the sun, and at one point 
thought he detected two green lines, less refrangible than 
it; but at several places he saw a complete hydrogen 
spectrum (including C), which he attributed to prominences, 
until he was informed by Captain Tupman that there was 
no prominence near the slit. More proofs of the terrestrial 
nature of this portion of the corona, I think, taken in con- 
nection with the fact that the dark moon gave identically 
tlie same spectrum. It would appear that there was so 
much atmospheric reflection in Spain, and here and there 
at Syracuse, that the true coronal spectrum with its line 
near E, the existence of which we must now accept as 
established beyond all question, was partially masked by 
the prominence spectrum with its usual well-known lines. 
There is one passage in Mr. Perry's interesting letter in 
which, if there be a misprint, as I suspect there is, an ob* 
servation of great importance is recorded. It runs, " Mr. 
Abbay, observing at Xeres with a spectroscope of 2 prisms 
of 45" belonging to Professor Young, saw the bright lines 
C, D, F ; and afterwards F and a line rather more bright 
than F on the less refrangible side of B, C not noticed 
then.'* Now, if b (not B) was intended here we have sub- 
incandescent hydrogen mixed with the grccn-linc-giving 



CHAP. XVI. 



The green 

coronal 

line seen. 



The dark 

moon ga7'e 

the same 

spectrum 

as a pot - 

tion of the 

cofona. 



268 



SOLAR PHYSICS. 



CHAP XVI. substance, which may probably be a new element with a 
vapour density less than hydrogen. 

So that roughly we might regard the chromosphere to be 
built up of the following layers, which are in the orders of 
vapour density in the case of known elements : — 

Substances X' (new element) Green coronal line. 

/// the t Sub-incandescent F. 

eoropia. Hydrogen J 

( Incandescent . . C, F, near G, h. 

X (new element) Near D. 

T\/To^««o;,,rv. f * and lines in blue and 

Macrnesium { 

"* 1 violet. 

Sodium D 

Barium Several lines. 

Iron, &c . . . . I Several lines, including 

The foregoing table excludes naturally the substance or 
substances which give bright lines in the solar spectrum, 
which are at times visible in the spectrum of the chromo- 
sphere. I have ventured to suggest that the substance 
which gives the line in the green is a new element, because 
invariably I have found that in solar storms the chromo- 
spheric layers arc thrown up in the order of vapour density, 
and because all the heavier vapours are at or below the level 
of the photosphere itself. 
Thepoiati' With regard to the question of polarization, the parties 
zatwn of jj^ Sicily obtained evidence that the corona was radially 

the corona. ^ ' 

polarized, though Professors Harkness and Eastman ob- 
tained a result which they explain differently. Mr. Ran- 
yard, at Villamonda, and Mr. Peirce, jun., north of Catania, 
obtained identical results in favour of strong polarization. 
Hence the solar corona, accepting these observations, not 
only radiates, but reflects solar light to us. A careful con- 
sideration of this fact, taken in connection with the possible 
addition of a, so to speak, terrestrial corona to its light 
may enable us to account for some of the obserx'ations, 
both polariscopic and spectroscopic, which .do not at first 



THE MEDITERRANEAN ECLIPSE, 1870. 269 



appear to harmonize with those to which I have referred, chap.xvi. 
notably those which give a pure continuous spectrum to 
the corona^ and which state that its light is only slightly 
polarized. 

From what has preceded, then, we seem justified in sug- 
gesting as working hypotheses the following, which, how- 
ever, more accurate information may alter, and which I 
offer as suggestions only, bien entendu. 

1. The Solar Chromosphere extends some 5' or 6' from 751^ work- 
the sun (Watson and others), its last layers consisting of "/f^^^^^l 
cool hydrogen (Mr. Abbay), and possibly a new element suHm^ 
with a green line in its spectrum (Young, Burton, and ^^J^^ 
others) ; which line, if it be identical with the auroral line ftof/s. 
as stated by Gould, may possibly be present in the higher 
regions of our own atmosphere. 

2. Outside this stratum the rays, &c., are for the most 

part due partly to our own atmosphere, partly to our eyes, 

for their shape varies ; they are seen by some at rest, by 

others in motion, and their spectrum is the same as that 

of the dark moon (Maclear). 

3. The white light of the chromosphere above the 
prominences, as seen in an eclipse, is due to its strong 
reflection of solar light, as shown by the polariscopic obser- 
vations (Ranyard, Peirce, jun., Ladd). 

4" The rosy tinge of the corona proper, that is of the 

''^ion more than 5' or 6' from the sun, is due to our atmo- 

^phere containing light which comes from both the higher 

3nd lower strata of the chromosphere (Peirce, sen., Maclear, 

^t>bay). 

^-BiNiCE, J^an, 9, 1 87 1. 



THE MEDITERRANEAN ECLIPSE, 1870 

{continued). 

II.— More Light. 

CH. XVII. In my former letter^ under the above title, written from 
Venice, I gave as shortly as I could the conclusions at 
which I had arrived as to the results of the various Eclipse 
expeditions, as gathered from the very imperfect informa- 
tion then at my disposal. Since I returned home I have 
naturally become possessed of more facts, though even yet 
the time has not arrived for discussing all the observations, 
as they must be discussed before an absolutely final verdict 
can be given. 

Still, there is so much general interest taken in the 
recent work, that I venture to return to it at the present 
time, more especially as I can now print a letter from a 
distinguished American astronomer, giving his view of the 
work done, and also as I am anxious to refer to Professor 
Young's article which has recently appeared in Nature} 
Professor Professor Peters, whose long and laborious researches <» 
^i^^ the sun are well known to all of us, thus writes in reference 
to my former article : — 

'' Its perusal has been to me a source not only of pleasure 
but of much instruction. You have placed on record, with 

^ Reproduced in the last chapter. 
^ This will be found in the Appendix. 



THE ME BIT ERR A NEA N ECLIPSE, 1 870. 27 1 



great lucidity, the question as it stood before the Eclipse, ch. xvn. 
and the points to be examined by the various ways of 
observation for bringing the question nearer to its solution. ^ pftn-l's 
Although the unfavourable state of the weather over the ^f^^^- 
entire zone of totality, as it seems, from Spain to Sicily, 
has greatly obstructed the execution of the plans and the 
extensive preparations made with the liberal .aid of our 
respective governments; and although hitherto, of course, 
only imperfect, mostly verbal, information has reached us 
of what the parties really did succeed in obtaining — still 
the result that is to be drawn from the sum total, as you 
are showing, seems of importance. The spectroscopic, 
polariscopic, and telescopic observations altogether agree 
in demonstrating an interior portion of the corona to belong 
to the sun. The existence of such a solar stratum is 
sustained also by my researches on the motion of spots 
when near the limb, pointing to a refraction on, or rather 
above, the sun's surface. I concur further in your opinion 
that the outer, more irregular radiating portion of the 
corona very likely owes its origin to our atmosphere. It 
is highly to be regretted that our Etna parties, in eleva- 
tions respectively of 3,100, 5,500, and 8,000 feet, suffered 
disappointment from a heavy cloud at the critical moment 
of totality. Their observations would have been decisive 
as to the local and atmo.spheric cause of the radiating 
coronal phenomenon." 

One more extract before I proceed. With reference to 
the suggestion (based on my observations of injections into 
the chromosphere) contained in my article, that probably 
the green line seen in the spectrum of the corona might 
indicate a new element lighter than hydrogen. Professor 
Young, claiming priority in the suggestion, writes : — 

"In Sillimafis Journal^ Nov. 1869, I wrote: 'Should What does 
it turn out that this line in the spectrum of the aurora j/y!'^^ 
does actually coincide with 1474, it will be of interest to sent f 
inquire whether we are to admit the presence of iron 
vapour^ in and above our atmosphere, or whether in the 



272 SOLAR PHYSICS. 



cH. XVII. spectrum of iron this line owes its origin to some foreign 
substance, probably some occluded gas as yet unknown, 
and perhaps standing in relation to the magnetic powers of 
that metal/ 

" This is the only reference I am able to make here. In 
my paper published in the Proceedings of the Amerian 
Association for 1869, the same thing is, I think, more forcibly 
expressed. I think you will also find it in my Eclipse 
Report in the 'Journal of the Franklin Institute' (and in 
my letter to Nature last spring). 

"The idea that 1474 might represent some new clement 
occurred to me at once when I found it in the corona, but 
of late I own I have more inclined to the opinion that it 
might possibly be a true iron line, and caused by meteoric 
iron dust of almost infinitesimal fineness ; yet I have alwap 
felt the difficulty of supposing the complicated iron spec- 
trum reducible to this one line."* 

I feel it due to Professor Young to give this extract, 
though I confess I do not see that the suggestions arc 
similar, nor do I see anything similar in the letter referred 
to, though I have lighted upon this passage which I had 
forgotten, which shows the great advance that has been 
Professor niadc. Profcssor Young last year wrote : * "It is not im* 
Youtt^'s possible that the so-called corona may be complex. Some 
portion of its radiance may, perhaps, originate in our oioJ 
atmosphere, although I do not yet find myself able tw 
accord with the conclusions of Dr. Gould and Mr. I/xAyer 
in this respect, and am strongly disposed to believe that lh« 
whole phenomenon is purely solar.'* His present \\t^ 
were given three weeks ago, as in the main concurring «itli 
my own. 

With reference to Professor Young's article, I am aDxiocs 
to say one word on the " sudden reversal into brightness 
and colour of the countless dark lines of the spectrum at 

* It will be abundantly shown in the sequel from Professor Younj* 
and my own researches that this line is not due to iron. 
^ AHtej p. 255. 



THE MEDITERRANEAN ECLIPSE, 1870. 273 



e commencement of totality," witnessed by himself and ch. xmi. 

r. Pye. I have seen this once, and only once, during 

. my observations, and Professor Young (who enjoys 

tter atmospheric conditions than I do) has never seen 

when working with the new. method. Now, I hold that 

e new method is competent to pick up such an envelope 

the one referred to by Mr. Longley, if it can pick up an 

rush similarly composed ; and although, of course, the 

pours competent to give such lines are not far off, as the 

Unary observations prove, I do not think they are ordi- 

rily high enotigh above tlie level of the photosphere to be 

n in this manner} That the number of lines is largely 

:reased when the atmospheric glare is withdrawn, was 

Dved during the American Eclipse. 

But to return to the corona, the main ^point of attack 

ring the last Eclipse. Since my last article was written 

have had an opportunity of inspecting copies of the The photo- 

autiful photographs taken by Mr. Brothers at Syracuse, fj^^^^'^/- 

d also one of the photographs taken by the Americans in /he corona. 

>ain. These, compared with the sketches taken at the 

spective stations, are very curious. In the Spanish photo- 

raph there is a very distinct *' rift," or dark space in the 

3ronal region, extending, I believe, almost to the sun, and 

linter indications of two other such rifts in another region, 

ot extending so low down in the corona. So far as the 

cts have yet been before me, only one of these rifts was 

'etched. Now, at Syracuse Mr. Brothers also photographed 

is— three rifts ; but the sketches did not record a single 

c. In Professor Watson's drawing, a copy of which I 

^e now in my possession, there is no indication whatever 

them. But there is a much more important fact behind. 

course, if these rifts had been in the same positions 
the two photographs, taken at stations so wide apart 

Spain and Sicily, the presumptive evidence in favour 
the solar nature of the corona for a distance outside 

This has been italicized in 1873. See the chapters on the Indian 
-"psc and Part II. 



274 SOLAR PHYSICS. 



cH. XVII. the sun equal to its diameter, would have been overwhelm- 
ing ; and feeling that here was a crucial test to apply to 
a question which has so long been debated, but never with 
such interest among the workers as recently, it was with 
some excitement that I found myself before these two 
photographs some little time ago with two American astro- 
nomers of eminence, for the purpose of endeavouring to 
settle the question. Suffice it to say that we came to 
the conclusion that the rifts were not identical, that the 
two cameras had not photographed the same phenomenon, 
although at first there appeared to be sufficient similarity 
to make the matter appear doubtful, and, unfortunately, 
the photographs vary so much in size, and the margin of 
the American one is so limited, that it will be scarcely 
possible to make a final comparison until they are brou^t 
to a common scale, and superposed the one on the other. 
I do not think it is surprising that rifts should appear in 
both photographs, supposing a non-solar cause were at 
work, for the corona between the rifts on Mr. Brothers' 
photograph looks like a very wide ray. 

Assuming, then, for the present that the photc^raphic 
evidence goes the way of all the other evidence — that, in 
short, the solar corona, including all its fantastic boun- 
daries, has been probably reduced from one, two, or thice 
solar diameters, to six, eight, or ten minutes, — I care not 
which,* — let us examine some of the details of the vaiioas 
observations. 

^ I beg here to give the actual words employed by Dr. FranUnd 
and myself in the communication to the Royal Society on the subject 
Speaking of the chromosphere, it was remarked, ^ The tenuity of ^ 
incandescent atmosphere is such that it is extremely improbable te 
any extensive atmosphere, such as the corona has bc^n imagiDeil ti 
indicate, lies outside it." — Proc. R, •$"., February ii, 1869. I ne« 
imagined that all the corona was non-solar. Again {Proc. R.S^S^ 
116, 1870), discussing the American Eclipse, I state that dieiluyw 
sphere includes the *' radiance " observed in the American Ed^K* ^ 
which radiance Dr. Gould wrote as follows : — ^ An examinatiooof tk 
beautiful photographs made at Burlington and Ottumwa . . . ttd a 
comparison of them with my sketches of the corona, have led me » 
the conviction that the radiance around the moon in Uie pictures w^ 



nitues. 



THE MEDITERRANEAN ECLIPSE, 1870. 275 

In Professor Watson's drawing, the intimate connection ch. xvii. 
►etween the higher and lower levels of the chromosphere 
including the portions not at present observed by the new 
tiethod) comes out in a very striking way. Mr. Seabrooke, 
t my request, made careful maps of the positions of the 
prominences before the totality commenced, and Professor 
Vatson made his drawing of the corona independently 
»f the positions of the prominences. On the homeward 
Dumey the map was compared with the sketch, and to 
tsc Professor Roscoe's words, "On comparing the two Coind- 
Irawings thus independently made, a most interesting series '^^^-Q^' 
>f coincidences presented themselves. Wherever on the portiimsof 
olar disc a large group of prominences was seen in Mr. ^^^^^ 
>eabrooke's map, there a corresponding bulging out of the />rimi 
orona was chronicled on Professor Watson's drawing ; and 
t the positions where no prominences presented themselves, 
here the bright portions of the corona extended to the 
mallest distances from the sun's limb." We may remark 
hat these coincidences show the excessive fidelity of the 

oring totality is not the corona at all, but is actually the image of 
rhat Lockyer has called the chromosphere. This interesting fact is 
idicated by many different considerations. The directions of maxi- 
itim radiance do not coincide with those of the great beams of the 
orona ; they remain constant, while the latter were variable. There is 
diameter approximately corresponding to the solar axis, near the 
xtremities of which the radiance upon the photographs is a minimum, 
rhereas the coronal beams in these directions were especially marked 
tiring a great part of the total obscuration. The coronal beams stood 
1 no apparent relation to the protuberances, whereas the aureole seen 
pon the photographs is most marked in their immediate vicinity. . . . 
vhatever of this aureole is shown upon the photographs was occulted 
r displayed by the lunar motion, precisely as the protuberances were. 
The variations in the form of the corona, on the other hand, did not 
eem to be dependent in any degree upon the moon's motion. The 
tftg'*^^*' s^d elegant structural indication in the special aggregations 
f light on the eastern side may be of high value in guiding to a 
intlier knowledge of the chromosphere. They are manifest in all the 
liotographs by your parties which I have seen, but are especially 
narked in those of shortest exposure, such as the first one at Ottumwa. 
n 9ome of the later views they may be detected on the other side of 
he sun, though less distinct ; but the very irregular and jagged out- 
ine of the chromosphere, as described by Jansscn and Lockyer, is 
;xhibitcd in perfection." 

T 2 



SOLAR PHYSICS. 

drawing, and make it one of the most valuable of the 
products of the Expedition. 

On former occasions the corona has been stated to 
assume a roughly four-comered form. This was also 
observed in Spain last December, and seems at last ex- 
plained by three drawings made by one of the Amerion 
party there. 

At the commencement and end of totality, when tbe 
moon unequally covered the sun, tbe photographs have 
recorded an excess of light on the corona on the side 
' where the limbs occur nearest in contact I am told that 
this effect in one of Lord Lindsay's phot(^raphs is vei>- 
striking ; it is certainly so in one of Mr. Brothers'. In 
the drawings we have a slightly different eflTecL At the 
commencement of totality, when the western or right-hand 
limbs were in contact, we get Fig. 107(1); attheendof 




Flc. ID7.-Why tbe cororH wii ohwrvcd <qiurc. 

totality the appearance recorded was like 2; the picture 
at the middle of totality compounding both these appeaf- 
ances, and being roughly represented by 3, in which the 
rectangular appearance comes out in its full strength. 

A word now about the polariscopic observations. I mar 
remark on this that it is much more easy for us to explais 
slight polarisation which might be atmospheric, than it ■) 
to explain, if we content ourselves with laboratory expen* 
ments, strong radial polarization which must take place 
at the sun. If we assume that gas or vapour of coosidtf- 
able tenuity does not reflect light (although I think thi> 
is to assume very much for the gas or vapour at tke a*. 



THE MEDITERRANEAN ECLIPSE, 1870. 277 



at all events), what is it that reflects light to us at the ch. xvii. 
sun, and reflects it apparently only above the level of the 
intensely incandescent hydrogen ? Certainly not solar 
spray. If we deny reflection to gases altogether, may it 
not be the continuous portion of the spectrum of the gas 
itself to which the light is due ? But this question of polari- 
zation is certainly one in which very much remains to be 
done, and it is consoling to know that the results obtained 
now will much facilitate the planning of the next polari- 
scopic campaign, which, we may add, should not be deferred 
beyond the end of this year. 



THE STORY OF THE CORONA IN COl 
TION WITH THE MEDITERRAi 
ECLIPSE} 

CHAP. My present duty, a pleasant one, although it is ting 
xviiL^ a certain sense of disappointment,' is to state the c 
tions which were made of the recent eclipse in Spj 
Sicily, to connect them with our former knowledgej 
show in what points our knowledge has been ex 
In these observations, we had nothing to do with 
as ordinarily visible, but with the most delicate phent 
which becomes visible to us during eclipses. I i 
the corona. 

General Notions of the Corona, 

Let me, in the first place, show what is meant 
term, and state the nature of the problems we had 
us. I have here some admirable drawings, whicli 
show by means of the lamp, of the eclipse tl 
observed in 185 1 by several astronomers who left E 
in that year to make observations in Sweden, wh 
eclipse was visible. It must be borne in mind tl 
drawings I shall bring to your notice were made 
same region, at places not more than a few miles 

* A Lpcture delivered At the Rnval Institution nf Cmt 



THE STORY OF THE CORONA. 



The first drawing was made by an observer whose name is chap. 

a sufficient guarantee for its accuracy — I refer to Mr. '"'"" 
Carrington — and wtien ilie sky was absolutely free from fhe 

clouds. In the next diagram you will see the corona is ^"J?"'^ 

changed. The bright region round the sun is no longer lonna by 

limited to the narrow border of light round the dark moon, -^f- t"ar- 

as seen by Mr. Carrington, but it is considerably expanded, andihr 

Astrcncmir 
Royal. 




The third gives still a greater extension, although that 
picture was drawn within a quarter of a mile of the place 
where Mr Carrington's was taken. And lastly, we have a 
'rawing made by the present Astronomer Royal, of that 
*mc eclipse, through a cirro-stratus cloud, as unlike Mr. 
-■rrington's as anything can possibly be. So that you see 
* began with a thin band of light about the moon, which 
fOuld make the corona a few thousand miles high, and we 
'd with a figure which Mr, Airy graphically likens to the 
amcnt round a compass-card, and which gives the 

™6rTcil to was made by Peltcrson, al (jOtlcnburg ; the third by a 
■nd of the Rev. T. Chcvallier, al the same place ; and I might have 
>ed another by Feamlay, lalten at Rixhoft, in which the corona is 
[Set than in any of the others. The series is most instructive. See 
nK. R. A. S^ vol X " 



SOLAR PHYSrCS. 

corona a height equal to about once and a half the su 
diameter. 

I will next bring before you some drawings made dtir 
the eclipse of i85tj, which was not observed in Europi 
regions, but in South America by two first-rate observer! 
one, M. Liais, a French astronomer, who was stationed 



Draiaingj 

ofthi 

Seutk 

Ammcan 

«Upuof 

1858. 




Otmos, in Brazil ; the other, Lieutenant Gilliss, who ' 
also there as a representative of the American GoveninH 
and observed some thousand miles away in Peru. 

I will throw on the screen the appearances observnl 
these gentlemen, and I think you will acknowledge t 
same variations between their results, as to d^rce,"*' 
in one case we get a perfectly new idea of phenomeu' 
difference in kind, I would especially call attention in t 
Olmos drawing to those extraordinary bundles of rap 
wonderful shapes, which you sec are so much brightcrtk 
the other portions of the corona. Such forms have b< 
seen in other eclipses, but they arc somewhat lait 1 
drawing made by Lieutenant Gilliss bears the samerclil 
to that made by M. Liais as Mr. Carnngton's did to 



THE STORY OF THE CO ROSA. 



2S1 



Astronomer Royal*s; so that we may say that we not 

only get variations in the dimensions of the corona as 

seen at different stations, but that we furthermore get 

a strange structure introduced now and then in our 

drawing in r^ons m-here absolutely no corona at all 

exists in the other. 

So much by way of defining the phenomena and giving 

an idea of the eye observations generally. 

ijtX, me now attempt to show you how the phenomena 

oZ>served in the last eclipse bear upon the results which 

hsL<^ been previously accumulated by means of telescopic 

drm<d naked-eye observations, and by means of the polari- 

soo^pe and spectroscope. 



tHAf. 
XV II I. 



X. Telescopic and Naked-Eye Observations. 

a, — A Part of the Corona is undoubtedly Solar, 

The first use I propose to make of the telescopic and 

i^^i-1<ed-eye observations of last year, is to show you a 

P*^ otographic copy of an admirable drawing made by Mr. 

^ *^^tt, who, though unfortunate enough to see the sun only 

'^^^ a very short time, was yet sufficiently skilled to make 

8^^^^-^ use of that brief period. This drawing will bring 

"^^ fore you the fact, that even when a large portion of the 

^^^ wi remained unobscured by the moon, Mr. Brett was 

^^^«ibled to see a dim ring of light round the unobscured P^/^i^**^^/ 

rw__ . i-i* 1 t«» « til '^<' corona 

*"^^^rtion, which smce the year 1722* has been acknowledged, kavebfen 
"^^^yond all question, I think I may say, to represent some- ^"f ^^'^ 
^^\vi% at the sun. It was observed in 1722 round the un- wasun- 
^^^lipsed sun, and in more recent times by Mrs. Airy in '*'^'>J^- 
^ ^42, and by Rumker i^ minute before totality in i860, 
'^^Dt to mention other instances. Therefore, we have one 
^^iDservation made during this eclipse, confirming the old 
He, that in the corona there is a region of some small 

* For this and the following points the admirable chapter on 
■Eclipses in Grant's " History of Physical Astronomy" should be con- 
^viUed ; as also Madler's review of the obsenations made in i860. 



SOLAR PHYSICS. 

breadth at all events which is absolutely solar, and whid 
it only requires a diminution of the solar light to enable 
us to see. This, then, we may look upon as the known; 
now let us feel our way gradually outwards. 

b. — Rays, or Streamers, are added at Totality. 

The drawings made in all the eclipses which have been 
carefully recorded bring before us quite outside this nar- 
row, undoubtedly War region, observed before totality, as I 
have shown, and also by Mr. Carrington and byLieuteoast 
Gilliss during totality in 1 85 1 and 1858, extraordinaiy ap- 
pearances of a different order. While in fact we have a 




solar ring from 2' to 6' high, we have rays of all shapes and 
sizes visible outside, in some cases extending as far a$4> 
and in all cases brighter than the outer corona on whicb 
they are seen, the rays being different in different eclipsA 
and appearing differently to different observers of the WK 
eclipse, and even at the same station. Here is a copyiif 
a drawing made by M. Rumker of the eclipse of 186* 



THE STORY OF THE CORONA. 



and I show [t for the purpose of calling your attention to 
the fact that the two curious rays represented in it belong 
to a different order of things from those which we see in 
the rest of the corona. From the beginning to the middle ^ 
of the eclipse the east rays were the most Intense. In the 
next drawing, which was made by the same observer, you 
see something absolutely new: and now the western side 
of the corona is the most developed ; we have a new series 
of bright rays, and altogether it is difficult to believe that 
it is a drawing made by the same observer of the same 
■eclipse. 

The third drawing is a representation of the same eclipse 
by M. Marquez, who observed with a perfection of minute 
care which has scarcely ever been equalled ; I bring it 




Kbefore you to show that the rays he saw were altogether 
• differently situated. We may conclude, then, that the 
Irays, although extremely definite and bright — as bright or 
•brighter than the other portions of the corona which are 
[■visible before totality, they being im'isible before totality — 
(appear different to different observers of the same eclipse, 
land to the same observer during different phases. 



284 SOLAR PHYSICS, 



CHAP. 
XVIII. 



c, — They change from Side to Side. 

I have already said that M. Rumker observed that from 
the beginning to the middle of totality the rays on the east 
side of the sun were longest and brightest, and that from the 
middle to the end of totality the rays on that side of the 
sun where the totality ended were longest and brightest. 

We will now carry this observation a step further, by 
referring to three drawings made by M. Plantamour in the 
same eclipse, that of i860. In the first drawing we have 
the beginning of the total eclipse as seen in the telescope: 
with the naked eye naturally we should get the sun disap- 
pearing at the east or left-hand side, the moon moving from 
west to east ; in the telescope things are reversed, and we 
have it right instead of left : and here we have the same 
Changes thing that M. Rumker observed, namely, that when the 
(/uring eastern limbs were in contact, bright rays (M. Plantamour 

progress of » o ^ \ 

an eclipse, saw three) were visible on the side at which the contact 
took place. When the moon was half-way over the sun, 
two rays of reduced brilliancy were observed on that side, 
not necessarily in the same position as tfw^ first obsen^ed, 
but one of these has been abolished altogether; and on 
the other side of the sun, where totality was about to end 
we have three rays gradually suggesting themselves : at the 
end of totality the rays visible at the commencement arc 
abolished, and now, instead of them, and of those seen at 
the middle of the eclipse, we have a bran new set oi 
rays oh the side of the moon from whence the sun is about 
to emerge. 

This observation, I need hardly say, is of considerable 
importance in connection with the fact that from the ycaJ 
1722 almost every observer of a total eclipse has stater^ 
that there is a large increase of brilliancy, and an i 
of the size of the corona on the side where the sun 
just been covered, or is just about to emerge. 

Now, what was there bearing on this point in the rece«»^ 
observations ? I have here three drawings, which, thoup* 






THE STORY OF THE CORONA. 

roughly done, you will see are of great importance side by 
side with those of M. Plantamour. These are drawings 
which have been sent in to the Oiganizing Comtmittee by 
Mr. Gilman, who lives in Spain, and who took considerable 
iaterest in the eclipse, and sent the results of his observa- 
tions to England with the eclipse party when they came 
home ; and it is of importance that you should see every- 
thing that Mr. Gilmao has done. If you agree with this 
explanation of the square form of the corona, which was 
olMcrved in Spain this year, it will explain the quadrangular 
form observed in the corona in a good many previous 
eclipses. Mr. Gilman says that at the commencement of 
totality — let me remind you, the commencement was 
<letermined by the disappearance of the sun at the east 
iinib of the moon, which is east in Mr. Gilman's drawing, as 
he was observing with the naked eye — the commencement, 
he says, was determined by the corona flashing out very 
much like a capital D- You see on the black-board 
C'cactly the outline, and you will at once mentally associate 




°**^half of the diagram with the rays observed by M. 
•sntamour, and the other half, in which there is a nearly 
I*^»fect ring of light round the moon, with the corona 
**^served by Mr. Camngton all round it in a cloudles.4 sky. 
'^-t: mid-eclipse, Mr. Gilman also observed the corona, 
^•*etched out its outline carefully, and found rays coming 
^'Jton the opposite side, adding themselves on to the per- 
''ectring first seen there. Opposite the two salient angles he 
**^served at the commencement of totality — represented by 
^ne top and bottom of the upright stroke of the capital D 



286 SOLAR PHYSICS. 



CHAP. — there were two others ; the corona now appeared square, 



XVIII. 



and then, just before the end of totality came on, the two 
corners first seen were observed to disappear altogether, 
leaving^ nothing but a perfect ring, and where, at the begin- 
ning of the eclipse, nothing was seen but a perfectly round 
ring, the two exactly similar forms on the opposite side 
shot forth, and you got a D reversed (Q). Mr. Warrington 
Smyth, who drew a square corona, saw the light flash out 
into the corona before the end of totality, and believes 
that all the angles of the square were not visible at one 
and the same time. 

Here, then, you have observations of exactly the same 
character as those of M. Flantamour, to which I have 
referred. In the drawings of both are shown the inna 
part of the corona, which you saw growing in the obser- 
vations of 185 1, to which were added the strange forms 
observed in 1858. You have these strange variations 
positively growing at the same place, and the same time, 
in the same and in different ^y^. Obviously there must be 
very much that is non-solar — call it personality, atmospheric 
effect, or what you will — connected with it. We have added 
to the stable the unstable. The question is, to what is this 
unstable portion due ? 

rf. — They are very variously represented. 

I will now refer to other drawings of the late eclipse, 
which were made in Sicily. For some reason or oUier, 
which I do not profess to understand, the corona, wliick 
appeared in Spain to be square, and to Mr. Gilman likeaQ 
at the beginning, and like a D reversed (Q) at the end,— 
to all those with whom I have conversed who saw it ii 
Sicily, it appeared as round as you see it here, in this 
drawing made by Mr. Griffiths; and, instead of hoDg 
square, we had sent to us all sorts of pictures, a large 
number of them representing a stellate figure. Here is a 
drawing made by a Fellow of the Royal Society, on boird 



THE STORY OF THE CORONA. 287 

me of Her Majesty's ships (the Lord Warden) which chap. 
vere trying to save the poor Psyche at Catania. In this ^^"'^ 
vc have perfectly regular rays drawn from every region of 
:he sun, some long, some short, but similar rays are almost 
nvariably opposite each other ; but in the interior, inside 
liese rays, the corona is just as it was observed by Mr. 
jrriffiths at Syracuse. I now show you a drawing made Anexces- 
yy an American gentleman at sea, between Catania and ^^ ^ay^^ 
Syracuse, with one ridiculously long ray, a ray as long as observed. 
Bras seen by Otto Struve in i860. Other drawings were 
cnadey even on board the same ship, so unlike each other, 
uid so bizarre^ that I need only refer to them as showing 
that there at all events must be some personality. We 
liave, then, to account for the variations between the obser- 
vations made in Spain and those made in Sicily. I regret 
Jiat we have not a third order of difficulties to contend 
Ririth, as doubtless we should have had if observations had 
3cen made by Mr. Huggins' party in North Africa. 

e. — The Rays are accompanied by a Mass of Light, 

These changes of the rays from side to side are accom- 
[>anied by, and are perhaps to a certain extent due to, the 
bursting forth of brilliant light in their neighbourhood, 
where the limbs are nearest in contact. This was first 
observed by Miraldi in the eclipse of 1724, and has 
frequently been recorded since. Mr. Warrington Smyth, 
to whom I have before alluded, states that he noticed this 
in the last eclipse, and the photographs, I think, have 
recorded it ; but as there is some uncertainty on this point, 
I need only suggest it. 

f. — Long Rays are seen extending from tJie Cusps before 

a^td after Totality, 

So far I have referred only to the rays visible during 
totality, but long rays were seen, when a crescent of the 



288 



SOLAR PHYSICS. 



CHAP. 
XVIII. 



sun was visible in i860 and 1868, by Mr. Galton and Mr. 
Hennessy. Mr. Brett caught the same phenomenon last 
Kays from year; but as the sky was cloudy, the commencements of 

no/soiar ^^^ ^^^^ ^"'^ "^txc Seen, appearing like delicate brushes 
in prolongation of the cusps. These observations are of 
great value, as no one for one mofncnt imagines ikat thcsi 
rays are solar^ and yet they are very like those seen during 

totality. 



g. — Sometimes Dark Rays^ called Rifts^ are 

of Bright ones. 



These rays to which I have referred are, however^ ttot the 
only kind of rays that are observed. At times aiSLtBQi»as 
it were, openings in the corona ; the openings beill|g of die 
same shape as the rays, that is, expanding as thtylowrlhr 
dark moon, and opening more or less exactly u. the nys 
do. Like the rays also, they are sometimes voy nMerous: 
in other eclipses they arc few in number. Let us t^ie the 
eclipse observed in India in 1868. Several dnwii^flnde 
there showed the corona as square as it was diawB~iB$paifl 
last year ; others as round as it was seen in Siritj^f ^ 
the eclipse was not observed only in India, it waa^dbipvcd 
at Maiitawalok-Kelee by Captain BuUocky and .ffclRhK- 
Whan, on the east coast of the Malayan PeniMRib ^ 
Sir Harry St. George Ord, Governor of the Straili.<J|iltk' 
ments.^ In the former place we had rifts expanding iKpdi)' 
as they left the sun — one forms an angle of 90^ liit^H^ 
of another being parallel — separating patches of 
which in some places extends 2\ diameters of the 
from the sun. 



' The probable origin of these rays will be stated in the sequel 
3 For the observations made by this party see the " Account of a 
Visit to the King of Siam," printed at the Government press at Sinp- 
pore, in 1868. The rifts are thus described: "It was noticed i&£ 
from several points in the moon's circumference darker raysemanatei 
extending to a considerable distance into space, and appearing Q* 
shadows cast forth into space by something not ver>- well defined." 



THE STORY OF THE CORONA. 



At Whae-Whan we are told that at one particular ckap. 
moment of the eclipse " it was noticed that from several "''"'• 
P9ints in the moon's circumference darker rays emanated, Tit dart 




iHctending to a considerable distance into space, and 
appearing like shadows cast forth into space by some- 
:hing not very well defined ;" these dark rays afterwards 
'diminishing." 




290 SOLAR PHYSICS. 

CHAP. Now let us pass on to the eclipse of 1869. In two 

xviit^ drawings made by Dr. Gould, in which the changes in the 

bright bundles of rays come out in a most unmistakable 

way, we get similar rifts, which changed as violently as M 

i'ariatioH the rays ) while in another drawing made by Mr, Gilnwn 

l/ihfnys. '^^ whole Corona is furrowed by narrow rifts in all regioas 




lying between violet, mauve-coloured, white, and ycIk'ttJi I 
white rays ! 

Now, what have we bearing on this point in the rccefi I 
observations ? No rift was seen in Sicily ; one rift w' 
recorded by the sketchers in Spain, but more than own'' 
was photographed in both places. We must rcmemlxr 




THE STORY OF THE COKOXA. 



however, in thus bringing eye-sketdies and photogc^ilis 
into comparison, first that the eye too often n sock ^ 
observations retains a genera) impresaaoa of Ae wbolc 
phenomenon, while the plate records the pheD o mea on as it 
existed at the time at which it -wis exposed ; and sccoodly. 
that we know that the plates record chcmicany. vtulc 




BPd uT tguUily. at BvfininB. 



tile eye records visually. We are dealing with two different 
Winds of light. 

I will show youtwophotographs on thescreen. Although Tie fMate- 
*lie lucid intervals were very rare, we were fortunate enough i^P^- 
*o get one photograph of the coronal regions in Syracuse, 
nd one in Spain. I now show you the photograph made 



J 



SOLAR PHYSICS. 



by the American party in Spain. You see here that, 
probably owing to a cloud, we get a certain amount of light 
driven on to the dark moon, and you also sec the indica- 
tions of the rifts. This photograph was taken with an 
instrument with a small field of view, so that the most 
important parts of the corona were rendered invisible by 
the instrument itself. 

Lord Lindsay, who also photographed in Spain, reconlcd 
no rifts. 

In the other photoj^raph, taken at Syracuse, the tesult a 
better. We have the L-qiiivnlfiit of tin; rift in the photo- 




graph I showed you before. The instrument was extrcmdy 
unsteady, and the definition not so good as it would \a\t 
been if Mr. Brothers had had a good opportunity of di»- 



THE STORY OF THE CORONA, 293 

ig his skill. We get other fainter indications of other chap. 
lere and there, and the question whether these rifts ^^'"' 

in the photograph taken in Spain with those in 
taken in Syracuse is one of great importance; and 
to be hoped that before long it will be set at rest. 
J observers think they agree ; others think they 
>t 

t there is an important consideration based on that 
»graph, to which I must draw your particular attention. 
'C shown you the photograph as it may be thrown 
e screen ; but in the photograph itself there are deli- 
letails which it is impossible to reproduce. The dark 
IBS in the corona indicated in the copy I have shown 
me merely the bases of so many dark wedges driving 
UHe> space, like their prototypes in the Indian eclipse. 

Brothers' opinion, I believe, that all you see on 
round the dark moon, all that enormous mass 

^Barly uniform in texture, and these beautiful 
between the rifts, are really and absolutely 

file solar corona. I confess I do not wish to 

inyself to such an opinion. We want more opinion 
the onus probandi lies with those who insist resen'ed, 

tit view, and I have yet to hear an explanation of 
that basis. 

ipfeJSiir Corona sometimes seems to be flickering or 
^V rotating, 

I^.ipoir come to the next point Time out of mind — 
ii^ for the last two centuries — the corona has been 
red to be flickering, wavering, or rotating, moving in 
' conceivable way and direction. In 1652 it was 
ibed as "a pleasant spectacle of rotatory motion."^ 
Antonio UUoa remarked of the corona observed in the 

>r many of these references I am indebted to Grant^s " History 
'sical Astronomy,'' which should be consuhe(i by the reader. 




294 



SOLAR PHYSICS. 



CHAP. 
XVIII. 

Corona 

observed to 

rotate in 

1652 and 

in 1788. 



N 



B 

Fio. 117 



eclipse of 1788, " It seemed to be endued with 
a rapid rotatory motion, which caused it to 
resemble a firework turning round its centre" 
The terms whirling and flickering were applied 
in the eclipse of i860. This extraordinary con- 
dition of things was also thoroughly endorsed 
by the late observations. It certainly exists, 
and is among the observations we have to take 
into account. When I saw an officer of one of 
the ships at Catania, I asked him if he had 
taken a drawing of the corona. " No," he said. 
I asked him, " Did you see any rays } '* " Yes." 
" Then why did you not make any drawing of 
them ? " His answer was, " How on earth could 
you draw a thing that was going round and 
round like a firework ? " This was not the 
only observation of the kind, and the tendenq* 
of such observations, I need hardly say, is to 
strengthen a belief in the unstable, and there- 
fore uncosmical, nature of these rays. 

Is this variation of light due to the bril- 
liancy of the corona, and the rapid change of 
the rays, which is one of the results whicki 
comes out clearest ? In 1 842 the brilliancy o< 
the corona was stated to be insupportable ^o 
the naked eye. A similar remark was made *o 
me by several of those officers who saw the h 
eclipse in Sicily. 



II. — PoLARiscoPic Observations. 

With regard to the polarization experiment 
by the kindness of Mr. Spottiswoode I am ett- 
abled to show you, in a very clear way, tic 
raisoH d'etre of the polariscopic obscn*ations 
made during this and former eclipses ; but tk 



THE STORY OF THE CORONA, 295 

ic ground is a wide one, and it is not my inten- chap. 
'er it to-night. ^^"'- 

had this arrangement of lamp, reflector, and 
ide, so that you may see how the polariscope 
mine the percentage of reflected light at dif- 
les, and the direction of reflection. Assume this 
"ig. 117) to represent the sun; let this reflector 
) the lamp represent a particle near the sun, 
light to the eye B: we shall naturally have 
reflected at a much larger angle than if the TheangU 
representing a particle in our own air were close of';^fi^l^^^ 
een. Having this idea of the angle of reflec- -whmthe 
jr minds, and the fact that the larger the angle r^^/^f^ 
>e conditions the more the polarization, if you near the 
lamp, as I have said, to represent the sun, and ^^ilgjjf 
r to represent any particle, of whatever kind 
e to imagine, it is clear that in order to get the 
polariscopic effect from that particle, you must 
situated that it will reflect light at a consider- 
to the beam coming from the electric lamp, 
is clear that, in order to polarize the beam most 
\ must place the reflector close to our imaginary 
so place it as to represent a particle in our own 
e, the angle will be so small that the polarization 
it will hardly be perceptible, 
our sunlight, which we will polarize at as great 
IS we can, by placing the reflector close to the 
sun, and send it through this magnificent prism 
Spottiswoode has been good enough to place at 
al ; and in the path of the beam I will place an 
that you may determine whether there is polarized 
•cperiment] You see there is considerable bril- 
those colours ; their brilliancy depending upon 
It of polarization, 
instead of having our reflector close to our 
sun representing a particle in the sun's atmo- 
.' place it near the screen to represent a particle 



296 



SOLAR PHYSICS. 



CHAP. 
XVIII. 

Small 

when 

moay. 



Thepdari- 

scopic 

results are 

doubtful. 



in our own, in which case the angle is extremely small 
the brilliancy of the colours will entirely disappear. You 
see it has disappeared. The colours, as colours, are dis- 
tinguishable, but their brilliancy has gone. 

That is the rationale of the polariscopic observations 
which have been made on the occasion of the last eclipse, 
with more elaboration than they ever were before. If wc 
found the corona to be strongly polarized, this was held to 
be a great argument in favour of the corona being a real 
solar appendage, an argument strengfthened if the polar- 
ization was also found to be radial. At present, however, 
a great many of the observations that have been made 
have not been received, and those that have been received 
are as discordant as those obtained in former eclipses, and 
therefore my account is an imperfect one, because I ha\'e 
not had an opportunity of discussing all these observations 
Indeed, if I had, I should hesitate to give an opinion on 
the subject. When Mr. Carringfton saw that small corona 
in 185 1, and Mr. -Gilliss saw that small corona in 1858, 
neither of them traced any polarization whatever; but 
when M. Liais saw that large corona in 1868 which i^-as 
invisible to Mr. Gilliss, he in his turn saw an immense 
amount of polarization, which led him to believe that 
the corona was solar, the whole of it, rays and evcr>'thing 
included, and that we had an indication of a solar atmo- 
sphere two or three times higher than the diameter of 
the sun ; that is, an atmosphere two or three millions of 
miles in height. 

This observation is not in accordance with the general 
conclusions from the drawings I have shown you ; and 
let me add that the assumption of reflection at the sm 
is not without its difficulties, and that wc have not yet 
traced reflected sunlight, even when the strongest polari- 
scopic effects have been observed.^ 

^ See the chapters on the Indian Eclipse of 1871. It is theit 
shown that M. Janssen has obtained evidence of the reflected sun- 
light. (1873.) 



THE STORY OF THE CORONA, 297 



III. — AlRY'S AND MaDLER'S CONCLUSIONS AS TO THE 

Results of the Pre-spectroscopic Observations. 



CHAP. 
XVIII. 



Before passing to the spectroscopic observations I will 
state the conclusions at which the Astronomer Royal and 
M. Madler arrived after the observations of i860 had been 
gathered together. 

The Astronomer Royal, in a lecture delivered before 
the British Association at Manchester in 1861, stated that 
the assumption of an atmosphere extending to the moon 
explained the observation of Plantamour, which could, he 
thought, be explained in no other way, and he held also 
that the polarization experiments seemed to show the 
same thing. The Astronomer Royal was content to find 
the reflection, which so many now insist must be at the 
sun, taking place somewhere between the earth and moon. 

M. Madler's verdict is in the same direction, and though 
he does not perhaps express so decided an opinion, he 
maintains that the atmosphere plays a principal part in 
the phenomenon ; and after detailing experiments to show 
this, he remarks of the solar and atmospheric portions, 
•* Both cover each other and unite in one phenomenon, so The corona 
that the corona is a mixed phenomenon." ^^ *^^^^^ 

I shall shortly show you that the spectroscope, leaving non, 
the telescope out of consideration, has taught us that this 
is true, though I shall not be able to show you that it is the 
whole truth ; we are not yet in a position to do that. 
Madler^ concludes his observations by remarking, "We can- 
not share the doubts of those who are afraid to surround 
the sun with too many envelopes ; neither do we find any- 
thing unnatural in the statement that the sun has as many 
atmospheres as Saturn has rings ; but we gladly admit 
that we cannot yet say anything positive. We have here 
a large field of probabilities, and the decision may yet be 
distant." We can speak with more certainty now ! 

* Sec Madlcr's papers in vols, xxviii. and xxix. of the Transacticm 
nf tki Jena Academy, 



298 SOLAR PHYSICS. 



CHAP. 
XVIII. 



IV. — Speci'roscopic Observations. 



a.— Spectrum of the Corona first observed by Tennant, 

Pogson^ and Rayet, 

We now come to the consideration of those observations 
in which we are aided by a most powerful and our most 
recent ally, the spectroscope, first used in the eclipsed sun, 
as you know, in the eclipse of 1868. You all know that 
in that year the question of the nature of red flames was 
for ever settled by M. Janssen, Major Tennant, Captain 
Herschel, and others, who observed that eclipse in the 
most admirable manner ; but we have nothing to do with 
the red flames now, we have to do with something out- 
side them. 

Now, most of you are under the impression, and it was 

mine until the day before yesterday, that the only thingwe 

learnt about the corona in the eclipse of 1868, was that 

The its spectrum was a continuous one ; and I need not tellany- 

^P^^j.^"'^ one in this theatre that the assertion that it was continuous 

0/ ine 

corona said was one that was extremely embarrassing, and implied that 
continuous ^^ ^^^ Something non-gaseous outside the red flames, 
which seemed very improbable to those who know any- 
thing about the subject. But some of you will no doubt 
remember that, besides Major Tennant, who made this 
observation, we had a French observer, M. Rayet, who 
gave us a diagram of the spectrum of one of the pio- 
minences, and Mr. Pogson, who has now been for some 
time in India, and is a well-known observer, who gave us, 
nominally as the spectrum of a prominence, a spectma 
with some curious variations from M. Rayet's diagram. 

I exhibit a copy of M. Rayet's diagram^ of the spectmtt 
of a prominence, as he called it. At the bottom is wfatf 

* The following is M. Rayet^s account of his observations : — 
*' La fente du spectroscope ayant ^t^ replace perpendicuJaireDCM 
au bord du soleil, c'est-^-dire, dans la direction de I'axe de la protnbf* 
ranee en forme de come, je retrouvai les neuf lignes brillantes vues 
instant auparavant. Toutes ces lignes prcnaient naissance au mtoe 
niveau vers la partic du champ spectral occup^ par rimage dc U 



THE STORY OF THE CORONA. 



299 



isidered to be the spectrum of the lower portion of 

ominence, while in the higher portion, where we get 

lines, as he considered, is the spectrum of the 

portion of the prominence ; the spectrum of the 



F h JS 

Fig. X18. — Copy of Rayet's diajin^in. 



jy 



portion contains the lines B, D, E, and F, and some 
lines, in all nine, while the spectrum of the upper 
f the prominence, as he thought it, only contains 
lines. It was at first difficult to account for these 
^tions. In the first place, one could not understand 
le B being given, because I soon found that the line 
not seen as a bright line in the chromosphere spec- 
it was clearly the line C that was intended. Hence 

nais du c6t^ qui r^pondait k la partie la plus ^lev^e de Tatmo- 
solaire, ou au sommet de la protuberance, elles se terminaient k 
iteurs variables ; quelques-unes d'entre elles se prolongeaient 
. de la longueur moyenne par un trait lumineux tr^s-faible. Cos 
s lignes diff^fraient d*ailleurs par leur dclat relatif. Le fac-simile 
(Fig. 118) d'un dessin fait peu d'instants apr^s P^clipse lorsque 
uvenirs dtaient tr^s precis, reproduit Tapperance g^n^rale du 
'• 

ipr^ la disposition des lignes dans le champ, d*apr6s'leur espace- 
slatif, leur couleur, et enfin, d'apr^s la physionomie mtoe de leur 
>le, j'avais assimil^ les neuf lignes brillantes de mon spectre 
incipales raies de Fraunhofer, B,* D, E, b, une ligne inconnue, 
nix lignes du g^oupe G. Cette estime ^tait imparfaite sans 
on verra cependant qu'elle n'^tait pas fort dloign^e de la 
— Mimoire sur les Rates brillantes du Spectre de rAtfnosphire 
etsur la Constitution physique du SoleiL 



CHAP. 
XVIII. 



I y a eu ici une erreur d'estime ; la ligne li ne fait pas partie du 
des protuberances, mais on y icncontre la ligne C." 



SOLAR PHYSICS, 



CHAP. 
XVIII. 

Observa- 
tions of 
Kaytt 



and 
Pogson. 



doubt was thrown on the other lines; it seemed as if M. 
Rayet was wrong about his elongated Unes D, E, and F, and 
probably meant C near D and F. And so it was explained 
— I am ashamed to say by myself — that there was no par- 
ticular meaning in these elongated lines, except that the 
spectrum of the prominence some distance away from the 
sun was simpler than it was nearer the sun, as happens in 
all prominences, as we may now determine any day ve 
choose to look at the sun by means of the spectroscope. 
Now let us hear Mr. PogsonJ He gave a diagram showii^ 

1 I here give his statement on this point in exienso, 
" A little before 9 a.m., when the partial phase was well advanced, 
1 turned the spectroscope upon the sun, and took the micrometer 
readings of seven known lines of the ordinar>' spectrum, agreeably to 
Mr. Huggins' instructions. 1 had previously taken care to have the 
equatorial stand in tolerable adjustment, and side by side with the 
Smythian telescope employed for the other class of obser\'atioii& 
After measuring the long prominence, as before described, about one 
minute after the beginning of totality, I directed the finder of the spec- 
troscope telescope to a part of the corona on the sun's southern Umbi 
as clear of any visible prominences as possible. A faint light was seen, 
scarcely coloured, and certainly free from either dark or bright linei 
While wondering at the dreary blank before me and feeling mtenscif 
disappointed, some bright lines came gradually into view, reached a 
pretty considerable maximum brilliancy, and again faded away. Fpc 
of these lines were visible, but two decidedly superior to the rest A 
turn of the right ascension tangent rod immediately brought back tk 
welcome lines, and by manipulating it with one hand, and the spcctio* 
scope micrometer with the other, the readings of the two brightest were 
secured. It struck me as strange that these brightest lines sboiU 
appear at a part of the spectrum not corresponding to any verr ooi- 
spicuous dark lines in the solar spectrum ; but not naving KircDhotf*s 
chart in my possession, I must leave it for my scientific friends at ham 
to decide upon the interpretation of the measures obtained. The thiid 
line seen, in order of brilliancy, must have been either coincident wik 
or very near the place of the sodium line D, but it was much fuller 
than the two measured ; while the fourth and fifth lines were extreneiv 
faint, and about as close as £ and f, but I estimated them to be soflK* 
where near the position of Fraunhofer's F in the solar spectrum. The 
fact of bright lines being seen at all, shows that the red prominence 
which produced them was composed of incandescent fas ; but wbete 
similar to any of our known terrestrial elements or otherwise, it vodd 
be premature for me to offer any opinion. The red prominence mdcr 
observation in my spectroscope was not the long one which most ockr 
observers naturally singled out for examination, but one of the t«* 
seen side by side about the S.E. by E. point of the moon's black disc 
I remcasurcd the seven ordinary lines of the solar spectrum about tk 



1 



lines in the spectrum of what he thought a promi- 
e, and he writes:— "A faint light was seen [in the 
troscope], scarcely coloured, and certainly free from 
\t dark or bright lines. While wondering at the 
ry blank before me, and feeling intensely disappointed, 



D Ef 

fit.. 119. -L'opy of pDgun'i dingranl. 

e bright lines came gradually into view, reached a 
ty considerable maximum brilliancy, and again faded 
y. Five of these lines were visible, but two decidedly 

■rior to the rest The readings of the two 

htest were secured. It struck me as strange that these 
htest lines should appear at a part of the spectrum 
corresponding to any very conspicuous dark lines in 

solar spectrum [These lines are a little less 

ingible than E.] The third line seen, in order of 
iancy, must have been either coincident with or very 
r the place of the sodium line D, but it was much 
(er than the two measured ; while the fourth and fifth 

I time on (he fullowing morning, and also rcpcnledly since my 
U lo Madras, the extreme difference in any case being four divi- 
t €»f the micromeiric scale for the lines A and C. The scale 
ttn (a mean of the [wo Mastilipatam readings) were as follows :— 

117 1513 in Ihe deep red. 
C IS47 in the bright red. 
D 1639 in the yellow. 
f;;«Jj both in .he green. 
F 1873 in the blue. 
, ('■ 20J5 In the deep blue. 



^ two bright lines were situated, the most conspicuous 
the other at 1763."— ^c/ar/ lif Ihi Gm-tiiimint AslreHomti 
I, Etiipst 0/ A ugnsl I %/h, 1 8M. 



743 




SOLAR PHYSICS. 



lines were extremely faint." [They were very faint and 
. DOUBLED, and near F. I have seen F give «-ay to a 
double line in our hydrogen experiments, though I am 
not prepared to say this is an explanation of Mr. Pogson's 
obser\'ations.] 

The fact that we have here the first observations of the 
spectrum of the sun's corona is one beyond all doubt ; and 
why M. Rayct and Mr. Pogson thought they were obscTv- 
ing prominences when they were observing above thecn, 
is explained by a remark made by Captain Tupman, of the 
Koyal Marine Artillery, who acted as jackal to Professor 
Harkncss, and picked out the brighter spots of the corona 
for his observation. Professor Harkness, observing the 
prominence bright lines, said to Captain Tupman, "You 
have turned the telescope on to a prominence; I want the 
corona." " No," said Captain Tupman, " I am giving you 
the corona as well as I can." It was certainly the corona 
ill both cases. Here you see, dimly and darkly, the first 
outcome of the spectroscope on the nature of the corona, 




THE STORY OF THE CORONA. 



303 



:ope in the bright-line region which had been spec- 
:opicaliy determined to exist all round the sun, and 
h, as in it all the various coloured effects are seen in 
eclipses, I had named the Chromosphere. It was 
• that by the new method of observing this without 
eclipse, by partially killing, so to speak, the atmo- 
ric light, we got a percentage only of the phenomenon, 
le atmospheric light could only be killed by an amount 
LSpersion which enfeebled and shortened the chromo- 
ric lines ; so that, although we could say that an 
lope of some 5,000 or 6,000 miles in height existed 
d the sun, we could not fix this as a maximum limit. 
her, when we examined the spectrum of this envelope 
rot long lines and short lines ; and I told how the 
t lines indicated a low stratum, and how a long line 
ated a higher one. To explain this, I will show you 
observation made long before the new method was 
ght of. Even before that time we had abundant 
ence of such strata, if we could not determine their 
re: we had distinct evidence either of one thing 
ning out, and then another, or that various substances 
; situated at different levels, under different conditions ; 
he first hypothesis, at the extreme outside of the chro- 
iphere the last thing would thin out, and then there 
lid be an end of all things as respects the sun. 
will show you a drawing made by Professor Schmidt of 
eclipse of 185 1. I do not wish to call your attention 
le strange shape of the large prominence, but to the 
that as the moon passed over this region we get a thin 
band, first along the edge of the dark moon, and after 
moon had passed over still further we see this red 
r, suspended as it were in the chromosphere, with a white 
r below it. This is the explanation of the long and 
t lines visible in the spectrum of the chromosphere : 
he red layer we have hydrogen almost alone ; below, 
•ed light was conquered by other light with bright lines 
.11 parts of the spectrum, and we get white light. 



CHAP. 
XVIII. 



The 

dispersion 

necessary 

to destroy 

the atmo- 

spheric 

light 

weakens 

and 

shortens 

the chro- 

mospheric 

lines. 



Schmidt^ s 
draun'tig. 



SOLAR PHYSICS. 



Lord Lindsay tells me he has a disUact mdication, wntteo 
, by the sun himself, that in one particular part of tlie chn>- 
mospbere, as recorded photographically in Spain, there 
were three such layers. And uver and over again wc find 
recorded white light close to the sun, then red alone, or 
rtd mixed a-ith yellow, then violet, and lastly green. And 
M. Madler remarks on this very admirably, " The violet 
band is the link betveen the prominences and the coroni" 

Before going further. I will show you the difference in 
the appearance of what we may term hot hydrogen and 
cold hjdrogen ; that is. hydrogen which we drive into dif- 
ferent degrees of incandescence by means of the sparfc. 
r After Dr. Franktand and myself were able to dctcnninc 
that the pressure in these solar regions was small, wc c»nie 
to the conclusion, that outside the hot hydrogen there 
must be some cooler hj-drogen, in order that the pheno- 
mena we obser\ed, both in the laboratory and in iIk 
ohservatorj-, should agree. 

I have in this tube hydrogen at a certain pressure, ind 




THE STORY OF THE CORONA, 305 



Tyndall, if he will be good enough, to observe the spectrum chap. 

of this hydrogen in this globe. [Professor Tyndall did so ] 1 

You will see that there is one line ? \Professor Tyndall, 
Yes.] And a continuous spectrum ? {Professor Tyndall, 
And a continuous spectrum.] Cool hydrogen gives us only 
the bright line F, plus a continuous spectrum, and many of 
you will know the extreme importance of that observation. 
It accounts for the F line being observed without the C line 
in 1868 and last year, and also for the continuous spectrum 
observed in the Indian eclipse. 

c. — The American Eclipse. 

• 

When we come from the Indian to the American eclipse 
with the considerations to which I have drawn your atten- 
tion-T-namely, the existence of these different layers due to 
the different elements and conditions of the same element 
thinning out — we shall see the extreme importance of the 
American observations, for they establish the fact that 
outside the hydrogen layer there was a layer giving only a 
line in the green, the line which Rayet and Pogson had 
observed associated with the hydrogen spectrum and the 
spectrum of the yellow substance. Here obviously we 
liave, I think, merely an indication of another substance 
thinning out, in spite of the extraordinary suggestion which 
put forward that the corona was nothing but a penna- 
solar aurora, 
I need hardly tell you that the idea of a permanent 
urora anywhere was startling, and that of a permanent 
^lar aurora more startling still ; but what I claim is that 
^^«ing last year's observations we made this very startling The outer 
a into a most beautiful fact— namely, that the outer ^^V' ^^^^'^ 
er of the chromosphere is in all probability nothing sj^iu^Tcm- 
e nor less than an indication of an element lighter ^'^f^ "^f^"' 

1 « ,-1 . . . . ** element 



|. ^«i hydrogen, although this is not yet absolutely estab- lighter 
J^^^f for the line is coincident with one of the lines in the , ^ ""' 
"•Mrum of iron. 



SOLAR PHYSICS. 



d. — The Layers increase very rapidly in Density. Refn- 
duction of the coloured Pfienotnena, 

Dr, Frankland and myself were early drawn to consider 
the solar nature of the large coronas, to which I have callal 
your attention, as extremely questionable, even on the 
supposition of cool hydrogen, because we did not sec h(W, 
with its temperature and pressure, it could extend \f[f 
far: and an experiment which I have to make here will 
probably make that clearer. 

We have in thesi; glass vessels hydrogen, a little nionr 
brilliant now the spark passes through it than that you saw 
in the globe, because I have been compelled to mix witl» 
it a certain amount of mercury vapour. Below, we have 
at the present moment sodium vapour being generated 
from metallic sodium in one tube, and mercury vapour ia 
the otlier. I hope, if the experiment succeeds, you will 
SLV that a good many of the coloured phenomena seen in 
tin- dirnmn^phere diiring cciipsi's may bL- easily reprotiiiO-'J 




THE STORY OF THE CORONA. 307 



v.— Conclusion. ^"^,^; 



x:eed now, if you will allow me, to some of the 

Lilts obtained during the last eclipse. 

that, although the work has been very unfortu- 

mipted, the result has been most satisfactory. 

\ together observations here and observations 

tisider our knowledge of the sun is enormously 

n it was a few months ago. For instance, we 

I to understand the long-neglected bbservation 

and the equally long-neglected observation of 

tid we know that outside the hydrogen there is, 

►ability, a new element existing in a state of 

lite tenuity. And we are sure of the existence cooi 

drogen above the hot hydrogen, a fact which h^^^gen 

be negatived by the eclipse of 1869. 

if we had merely determined that there was 
ydrogen, all our labour would not have been in 
ihows the rapid reduction of temperature ; but 
•re behind. I told you that M. Madler, in sum- 
he observations made up to i860, came to the 
that part of the corona was certainly solar, and Part of the 
er the outer portions were or were not solar, ^^^^"'^^^ 
er of doubt. I do not say that we have settled 
tely, but we have firm evidence that some of the 
e corona is due to reflection between the earth 
oon. The outer corona was observed to have a 
over the prominences, and the spectrum of the 
?s was detected many minutes above them, as 

the dark moon. It could not have got this 

he sun, for its intrinsic colour is green, and the 

r the hydrogen supplied at the sun is abolished 

is absorbed, and can only reach the corona at 

to speak, as dark light. 

reat fact that we are sure, as far as observation 
\s sure, that there is a glare round the hydrogen, 
{ us the spectrum of hot hydrogen on the corona, 

X 2 



2*r^ Is ~. 




«Che mA E^ to be caly 
B^K i^ ^fKkxA ft up e^i 
M^^ ■( GOBI ^rdRigcn u 
TepX it «Aere tbere i* BO 
MHB- Ami Ocnfttb 
Ei«B B Ae Sk «r the gnn 
m tt^ fat MBBfas amjr 
I «Ab twwv Mjrtbng zEnot 
K is GotH thA Ae demcDt 



^jts of Ae dan 



*rtl|iiliH;iiii1lliifirt 
sttsa ghic round 
ffcae gTfcs us a eiafc 
sto be caqiectcd, ID'S 
r::c b sU^gA^Hd bf the ofasHrmtiaa — aiDdSt 
ibf^rralitM aadc is Span— dot the air. dtf 
-r — -■g h tlBttj a ssaadlke d»k mooo. arttbc 
'.— 1-31 «c got finn tbe protiBBeaces theini«lt cs- 




THE STORY OF THE CORONA. 309 



; and therefore blue, higher up, the red and chap. 
ngling and giving us violet ; and then another _?Z!!!l _ 
nning out and giving us green. Take these 
connection with those which are thrown on our 
)r on the sea during eclipses, each region being 
ms with varying, more or less monochromatic 
that light of the very colour composing the 
jrs, each layer being, as I have shown, so much 
n the outer ones that its light predominates over 

too much to suggest to those who may be 
ittempt to elucidate this subject, that probably, 
Id consider all the conditions of the problem 
f that great screen, the moon, allowing each of 

by turn to throw its light earthwards — the in- 
F the edge of the globular moon allowing here Effect of 
\ from a richer region, here stopping light from ^4^. "'"^'^f ' 
nmer ones — they would be able to explain the moon. 

colours, variations, apparent twistings, and 
de? I do not hesitate to ask this question, 
is a difficult one to answer, since the whole 
►ne of enormous difficulty. But difficult though 
: I have shown you that we are on the right 
lat in spite of our bad weather, the observations 
J English and American Government Eclipse 
of 1870 have largely increased our knowledge, 
ease of knowledge generally comes a necessity Question 
: the nomenclature belonging to a time when ^^^PJl^^^"' 
rfect The researches to which I have drawn 
ion form no exception to this rule. A few 
ur science was satisfied with the terms promt- 
7, and corona, to represent the phenomena I 
It before you, the nature of both being abso- 
)wn, as is indicated by the fact that the term 
mployed, and aptly so, when it was imagined 
jnces might be solar mountains! We now 
of the constituent materials of these strange 
know that wc arc dealing with the exterior 



so£^M fisracs. 



:; xai a tu^ kncnrledgt d 
i alftad^ acqatcd, vlbcii sbowi lu the 
■^t' thoe pnaoKBas. But we alw 

the comma, is wt at the am at all 
H::=ce the i;r-,- .- 1 i^ufciy aad Aifc have been sDggbtcd 
to designate :: i!:± oae ^ne tike Rgrons vliere the gcoenl 

r2>i:i.:i-jti. o-^^;^ :: 2 i rAacKd p c es B ur e and tentpcnlurcu 
cj I:^tT £-^'i-.ri r_iie In the sd e ct i r e ladiatioD, and, to the 
Giher. thai pi:~. :f the coraoa which is ooo-scdar. Ncilbci 
of these terrn^ := ipt. Bor is ekher Bccessary. All purpoes 
vri:: be sen-ed i:' ihe tarn anm m be letatoed as a nunc for 
the exterior r^ioa, lacfaMEag the lays, lifts, and the like* 
abo-^t -shich doubt still adsts, though it Is now /nmt/ that 
somt pan is ntm-soUtr ; while for the undoubted solar po*"' 
tion the term Ckrffm»^KTf — the bright-line regioa— u »* 
K-as defined to this tiicalzc now two j'cars ago, cXiUlI>* 
expresses its characteristic features, aod differeDtiatcs »* 
from the photo^here and the associated portion gftfic 
solar atm.jisphere. 




THE ATMOSPHERE OF THE SUN> 



r is now as nearly as may be two centuries ago since a chap.xix. 
an, whose immortal fame has made of Cambridge a place 
pilgrimage, proved to the world by " reason and experi- 
ents" that " the light of the sun consists of rays differently 
frangible ;" and in the same book in which the results of 
* work are recorded, we find the following pregnant 
estion, which shows that he who had thus, all unknow- 
?ly, given us the means of discovering the truth, had 
"cady, at one bound, arrived at a conception of it which 
propose to show in the present discourse is right in the 
atin. 

" Are not," said Newton, in his eleventh Quer)% " the Nt-^ototCx 
n and fix'd stars, great earths vehemently hot, whose S^^'f^'^'* 
^t is conserved by the greatness of the bodies, and the 
^tual action and reaction between them, and the light 
■^ich they emit ; and whose parts are kept from fuming 
•^^y, not only by their fixity, but also by the vast weight 
^d density of the atmospheres incumbent upon them, 
^d very strongly compressing them, and condensing the 
•-pours and exhalations which arise from them ?" 
It should be a lesson to those high in place, who have 
in their power to encourage a widening of the boundaries 

The Rede Lecture, 187 1, delivered in the Senate House, Can\- 
idge, on May 24, 1871. 



312 



SOLAR PHYSICS. 



ll^olliis- 

ton^s and 

Fraun- 

hofer^s 

obsen'a- 

tions. 



cKAP XIX. of knowledge, and fail to do so, to reflect that the world 
had to wait for considerably more than a century for the 
next steps which brought Newton's work with the prism, 
and his Query, into closer connection, although the prism 
was competent to lead us to a discovery of as high an 
order of importance in one branch of physical astronomy 
as gravitation is in another. 

The steps I refer to were Wollaston's detection of the 
dark lines in the solar spectrum, and Fraunhofer s obseni- 
tions of the spectra of some of the fixed stars in which 
similar lines were detected. 

Here, then, we had the sun and stars linked together 
by still another bond — the spectra of both contained dark 
lines ; but the opinion of the time was, that far from being 
great earths vehemently hot, they were great earths cod 
and habitable as our own. 

Still another half-century was sacrificed to our imperfect 
organization in matters of science, and then the myster}of 
these dark lines was solved — solved in part, that is to say; 
and by a man whom this ancient University also daifl» 
and one who is still among you to receive the honour 
which is his due — I mean Professor Stokes — although tk 
discovery, which is the most important one of our agci* 
generally ascribed to Kirchhoff and Bunsen, as they *«^ 
the first to publish it. 

Now, I need not tell you that the first obvious out- 
come of this splendid generalization is the proof that tk 
sun and fixed stars are undoubtedly " vehemently hot; 
for if there is anything at all in spectrum analysis vW'* 
teaches us or can teach us anything about the sun ^ 
stars, we proceed on the assumption that we haW 
the sun and stars "vehement heat;" and the beauty «■ 
the reference to the "great earths" is further cnhan*' 
by the fact that, instead of finding the sun comp*' 
of substances which are unknown to us, many elclD^ 
which are most familiar to us earth -dwellers arc fo*' 
there. 



THE ATMOSPHERE OF THE SUN. 313 

rhat the sun and stars were great earths vehemently chap.xix. 
; was now, in fact, put beyond all question by the 
jm : for I need scarcely remark that not only were 

dark lines revealed by that instrument in the solar The sun 
ctrum now shown to be due to the absorption, by ^af/J^rThs 
indescetit gases and vapours, of light proceeding from vehemently 
ething more intensely heated still; but the presence '^^* 
lydrogen gas and the vapours of sodium, iron, mag- 
um, barium, and many other terrestrial elements in 
atmosphere, was absolutely demonstrated by the exact 
ng of some of the dark lines I have referred to with 
;ht ones in the spectra of the vapours of those elements. 
? sun and stars burst into song, and the whole heaven 

filled with new and exquisite harmonies. 
►ut Newton refers not only to a great heat, but to a 
city" of the " parts ;*' by which reference, I take it, he 
ws us that he perfectly understood that the heat of 

fixed stars and of the sun could not be the heat of 
ibustion, arising from chemical change, but the heat of 
indescence. 

n the theory of the sun based by Kirchhoff on his Kirchhoj^'s 
>eriments, the general surface of the sun — the photo- ^ '^'^^^' 
ere — is regarded as liquid, as one unbroken molten sea, 

continual motion, as our own are when raised by 
"nis and foaming waves ;" and these waves are ascribed 
fte enormous changes of temperature occurring in an 
flying solar atmosphere, and the force of the currents 
'ch must in consequence be produced. The atmosphere, 
^'tively cooler than this molten sea, is on this theory the 
ona, a phenomenon visible to us during total eclipses ; 

most effective absorbing layer being situate at some 
tance above the sun. The only "condensation" which 
'chhoff allows is that rendered evident to us by spots 
>ch according to him were clouds suspended in the 
losphcrc, and therefore situated above the photosphere, 
which the atmosphere, according to him, rested. 




Now Newton — as I hold is clearly evident from ihc 
latter part of his Query — had in his mind something vcrj* 
diiTi:rcnt from this — something which I hope to shuw is 
more in harmony with all the telescopic evidence, and with 
some recent work with that instrument of his, the prism, 
in which the attack on the sun has been varied from tlut 
hitherto employed. 

In the researches which culminated in the discovery 
' to which I have alluded, ordinary solar light was admiltitJ 
into the sHt of the spectroscope irrespective of the partfif 
tJic sun whence it came; the light even might have bcer» 
reflected from a cloud. In this way it is clear we dex-^ 
witJi the average of all the light radiated from all part-:^ 
of the sun, assuming that different parts of the sun do giv^S 
us li;.;ht of different kinds. But if by means of a tclcscop^^ 
we first obtain an image of the sun, and so arrange Ih- 
spectroscope that only the light of the portion of the ain 
tiie inin;Te of which is on the slit can enter it, then it l;^ 
clear th;a we shall be able to examine the spectrum (^ ■ 




THE ATMOSPHERE OF THE SUN. 315 

lipses — a region which has been named the Chromo chap.xix. 
here on this account, to distinguish it from the white 
lotosphere. With the new method employed without mosphere 

eclipse^ we find bright lines of different height in the isdistin- 
ectrum visible all round the sun ; and although the TrightUms, 
»:trum is generally much more simple than we should 
ve supposed, I have thus seen hundreds of lines re- 
nsed. There is generally no part of the sun's periphery 
icre they are absent ; the heights of the various- lines 
served change very slowly at times in sweeping over 
long arc, at others rapidly in sweeping over a short 
e,— over a region, in fact, where there is a prominence. 
ic shorter lines are generally visible only where the 
rher lines are highest, and are most numerous when the 
rhest lines are brightest. 

Now, what substances do these lines indicate ? 
rhe longest lines are due to hydrogen, as proved by rhehngest 
^ fact that they are prolongations of the Fraunhofer ^'"^^ '^'"' '^^ 
as C, F, one near G, and A, in the solar spectrum. The 
ct highest line, one in the orange, corresponds with no 
sorption line in the spectrum, and there can be little 
ibt that it represents a new element. The heights of 

other lines with corresponding Fraunhofer lines, when 
ble in the same prominence, are almost invariably in 
following order : — 

ifagnesium or Sodium, highest. Magnesium 

Nickel or Barium, next below. or sodium, 

ron and other lines, shortest. TariLZ 

Jut there are other lines thus visible at times, which andiron 
)roach the sodium level, and these lines are either bright "^^ *^'^' 
the solar spectrum itself, or have no corresponding 
lunhofer lines. Of this more presently. 
rhe number of the lines, and their height and brilliancy, 
iently depend upon some action coming from below, 
n the region of greater heat. 

Jow, in eclipses our knowledge is carried further, because 
are face to face with the phenomena, and no longer 



SOLAR PHYSICS. 



. have to pierce the br^ht atmospheric veil by which ihess 
regions arc generally hidden. 

In the first place, the hydrogen lines are no longer 
highest, nor are they all of equal height ; and in the nwt, 
the number of lines visible is increased ; both of whidi 
effects might be expected to follow from the improved 
conditions of observation. 

It is rendered evident, then, by these observations nilli 
the spectroscope, that we are here dealing with layin due 
to the thinning out of the different vapours : such layers, as 
Mr, Johnstone Stoncy has insisted upon and which Pro- 
fessor Pierce has shown, must exist in a mass of mixed 
vapours, so situated, under the joint action of diffusion and 
graiity. Noiv this is exactly the evidence furnished to 
our eclipse observers by the telescope. They have recorded 
nearest to the sun's limb a white region, the many lined 
layer in the spectroscope, next fgoing outwards) a yellow 
l.iyur. I lore we have lost most of the lines, and have only 
llie sLibitance which fjives the yellow line associated wilh 




THE ATMOSPHERE OF THE SUN, 317 



ic observation nor during eclipses do we easily and chap.xix. 
ously see all the dark Fraunhofer lines reversed ; and 
a they are reversed in great numbers, the tell-tale lines When 
:ate that the absorbing vapours which give rise to them ^f ^/^^* 
lot extend to any great height So that, in spite of the are re- 
er which the spectroscope gives us, we do not observe ^aZl%!nt 
he lines round the sun bright, which we get dark in layer is not 
light coming from his disc Does not this mean that ^*^' ''^* 
are not dealing with the whole of the solar atmo- 
are ? Let us consider this point. First, a difficulty of 
d^ation may come in the way : the atmospheric light 
::h we have to deal with in one case, and the asso- 
ed continuous spectrum in another, may be too strong 
us; or, again, it may be that the vapour which gives 
he majority of the Fraunhofer lines is so limited that 
instruments are not capable of picking up all that is 
ig on. However this may be, the fact remains that we 
lot easily account for all the Fraunhofer lines ; and the 
m tells us that the further we go from the sun the 
chance have we of finding them, since, when we get 
ie extreme regions of the corona, the spectrum, instead 
eing complicated, is one of extreme simplicity. In fact, 
chances are that the spectrum of that region only con- 
of one faint line, with a glimmer of continuous spectrum. 
ice, then, we must work nearer the sun in our search 
he reversal of the outstanding Fraunhofer lines. 
ow, on Kirchhoff's hypothesis the photosphere is a 
ten sea. If that were so, we should have no atmo- 
jre below its surface, and the surface would be so level 
we must find the atmosphere above it, which we do 
readily. But let us assume that the surface of the 
rospherc is not level ; then there is a possibility in favour 
ome such notion as this — that the photosphere, being 
dy or gaseous, has an uneven surface, with faculous 
es and domes, interspersed with cavities, which perhaps 
indicated by what are known to astronomers under the 
c of pores. This I maintain is in harmony with tele- 



soijut pmrsKs. 

5Cop:c obsen'atioii ; and if this be the stateofaSatrSiitim}' 

be that most of the absotpdoa 'ines which we get in the 
spectrum, which we cannot account for in the br^ht !ino 
of the spectrum of the regions abox'e the photospbcn;, nu)' 
be due to the absorptioa irtiich is going on in these [lUoi 
And here permit me to remind j'ou that the solar spcctnin 
' with which we are all so familiar is not the spectrum of 
any one part of the sun ; it is an average spectrum,— a 
spectrum built up of all the spectra obtained by on exa- 
mination of everj' point of the sun. 

It must Dot be imagined that ewry part of the sun 
will give us such a spectrum as this. It docs not; and 
here we get great help in the point we are considering. 

What facts, then, have we to guide us in tlii-s matter? 
Let me place before you one or two^ 

What is seen when the spectroscope, instead of bcii>£ 
occupied with the light outside the sun's limb, is dealing 
witJi the light on different portions of his disc? — when in 
fiict we arc obaer^'ing the vapours between us and the sun- 




THE ATMOSPHERE OF THE SUN. 319 



rould, in a powerful telescope, give us eti petit what we chap.xix. 
low get tfi grand. These darkish bands running along 
he spectrum are due to small spots or pores. Where we TJu spec- 
observe a faculous region in the same way, instead of these i^htJ^^in 
larkish lines we get, as it were, bands of brightness, some- ^he regions 
lung very much brighter than the spectrum of the ordi- facuia. 
nary photosphere. 

We will now go a step further, so that another diflference 
which exists may also come out in its strongest form. Let 
lis observe a spot of some considerable dimensions. Here 
Jie indication of the continuous absorption which darkens 
he spectrum throughout its length is more obvious still. 

shall be able, I hope, subsequently to show you that this 
% a very valuable indication of particular conditions of this 
egion ; but we have now merely to note the phenomena. 
*o do this, let us limit our attention to the double line D, 
'le two dark lines indicating the lines of absorption due 
^ the vapour of sodium. Imagine that on the slit of 
ae spectroscope lies the image of the spot : we see that, 
* addition to the general absorption which is always to Selective 
^ found in spots, and which is absent when we are dealing af^sorption 

f5^i_ , . - , , .,11 *»* the spots, 

^«i a spotless portion of the sun, we have a considerable 
tokening of the absorption lines, which, in the case I 
^Vc taken, is due to the vapour of sodium. 

I might have taken other parts of the spectrum, con- 
^ning the iron and calcium lines, and the same pheno- 
menon would have been observed. Here, then, we have 
^ chain of facts : — 

(a) The spectrum of a facula is brighter than the spec- 
>^m of the ordinary solar surface. 

iff) Hence the ordinary solar spectrum has undergone a 
general absorption. 

(f) The general absorption increases gradually and con- 
nuously as the prism analyses the light of a spot with 
helving sides, until it is greatest in the spectrum of the 
nbra. 

(d) These variations in the general absorption are accom- 



9 



t CHaanaHip vdfc ^ jH^ gf 




THE ATMOSPHERE OF THE SUN. 321 

idous convulsions in the photospheric region. There- chap.xix. 

, in addition to the spectroscopic evidence, we have the 

lal evidence of the appearance, under a high telescopic The 

cr, of changes which can hardly go on except in a ^i^^ 

erial mobile to a great degree. Still, enormous as the spectro- 

« changes arc, I shall show you that they are as ^'^' 

ling compared with the changes that go on in the 

ler regions of the atmosphere — the regions above the 

tospheric layer. 

«t me give an instance, and refer to two drawings of 

same prominence, made at an interval of ten minutes. 

en I tell you the size of the prominence, and the time 

change took to register itself, it will be obvious to you 

we are dealing with a region still more capable of 
:hoing to the slightest force. The prominence to which 
ifer was 40,000 miles high. In the interval of ten 
utcs the forces at work were such, that at the expiration 
hat time scarcely a trace of its original structure was 

Even these changes are not the greatest that I might 
r to among my own observations, and, indeed, those 
thcrs ; for, fortunately, spectroscopes are getting more 
mon than they were, and observers in all lands are 
ing rich harvests of facts, and among them abundant 
ence of the point I am now enforcing — namely, that 
motions are most rapid and the changes most intense 
hose parts of the atmosphere where its constitution is 
Jest: in the photosphere we have marked changes from 
to day; in the chromosphere from hour to hour — I 
almost said from minute to minute. 
s we descend the atmosphere, then, we find the number 
s constituent materials to increase, the general absorp- 
to become more apparent, the lines to thicken, and the 
n to be less rapid. 

irther, we find the most perfect continuity wherever it 
)ssible to observe it. Hence we are justified in con- 
ng from the evidence of the chromosphere a complete 
nuity of the solar atmosphere from that portion of it 

Y 



Theitiidy study the Spectra of all spots as carefully as the 

"v-Ul eddto °'' '^^ Kcneral surface of the sun has been stud 

our knavi- look- out for the deepest spots, and if it should i 

^mvr ' ^^ ^'^ have already grounds for supposing, that ; 

iiraia. cular point in the sun-spot period the spots are d 

they are at other times, and if we are careful to 

diligently, then ive shall be able to carry our rec< 

solar atmosphere very much lower, and discover I 

are not included in our observations up to the pri 

We may already guess in what direction disi 

lead us. Dealing with known elements, we ha 

all invariably hydr<^en ; then, next, a layer of n 

and sodium; lower still, of iron, calcium, and 

we take these facts in connection with the othci 

we have not the heaviest metals writing their re( 

solar spectrum, may it not be that the reason i! 

vapours of those metals are so far beneath th 

brethren that they are never thrown above the pi: 

by which the spectrum is reversed ? 

NewioH Newton, in his Query, suggests not only " vap 
^aiLas'. " exhalations." Of the vapours we have alrea 
something ; the lines in the solar spectrum arc thi 
the existence of them, but what do wc know ab 
exhalations, in which term wc get a dynamic not 
duced ? Do they exist .' and if so, is the exhalati 
like the upsoaring of our own clouds, or fierce aiii 
as we mipht easily imaf^ine it to be in suf:h asuri 



THE ATMOSPHERE OF THE SUN, 323 



But I venture to think there is a little doubt connected chap. xix. 
with this method of grappling with the subject. I question 
wliether we have here the best way of determining the indUatedby 
presence and movements of the exhalations, but we have .^ change 
a very undoubted one, which I need not demonstrate in ^*\ion of' 
this Senate House, which depends upon this consideration : ^pptrai 
if we have any of the vapours on the sun moving with a 
velocity comparable with the velocity of light, the selective 
"^diation and absorption of those vapours will indicate the 
"Motion. In other words : — their absorption and radiation 
^hen in motion will not be the same as their absorption 
and radiation when in a condition of rest. 

The spectroscope has enabled us to come to the facts 

"* all these cases; but in order to bring them entirely satis- 

^crtorily to your notice, it will be necessary to say a few 

^^c>rds about the language in which the records are written. 

Such changes of wave-length give rise to what, in the 

c^-se of the prominences, I have called motion-forms. 

Those are not the forms of the prominences themselves 

^*^liich we have on the slit, but forms due to the changes of 

'■"•^frangibility, which deflect the bright or dark line under 

ocamination now towards the violet, now towards the red, 

and now towards both: both on and off the sun they shiver 

tVie perfect image of a perfect slit, now into hard crooked 

Unes, and again into most delicate films. 

It is clear that when such observations as these arc made Difference 
at the sun's edge, we are not dealing with Newton's exha- f*'^*^*^" 

I • . 1 1 r 1 changes of 

«tions, because any motion on the edge of the sun must 7£wr- 
^ a sort of wind-motion on the sun. But the same facts /^^^f^w 
are to be gathered from the centre of the sun itself, where limb, 
^f course any change of wave-length must be due to 
"lotion in the direction of the line of sight, and must 
"^crefore be an indication of an up or down rush. Now 
^^ the sun we have this further evidence in our favour, 
^t almost invariably when the line due to any vapour is 
^fleeted towards the violet, that vapour no longer plays 
^ part of an absorber ; it radiates and appears bright, 

V 2 



hidieaiioH bright line indication ol an uprush, we get an al 

rHshl'u'hc ''"^ ^y ^'^^ ^''^^ ^^ '^' °^'^" moved towards tlie rt 

•■(HireB/ shows that we have relatively cooler hydrogen dt 

' '""■ from above the disturbed part, but at times the ir 

in many cases disappears altogether; that is, we 

longer relatively cooler hydrogen — the whole of t\ 

incumbent hydrogen has been heated to the same 

ture as that of the newly-ejected material, which i 

or at times hotter than the photosphere. This I 

we have still another argument for the continuil 

solar atmosphere to the regions beiow the nivea 

photosphere, where we must look for Newton 

atmosphere, and not above it, and here is proof ; 

the deeper we go the hotter we get. 

There is still another point to which I should li( 
attention for a moment. We often find somcth 
like the lozenge again in connection with small sp 
I would also call attention to the disappearanc 
hydrogen dark lines, in connection with its appears 
the way in which in small spots we get the hotter t 
rising up. 

Again, we may get this lozenge associated witl: 
plete brightening of the hydrogen near it, which 
at rest, which is proved by the fact that the neigl 
portion of the absorption line more or less suddenly 1: 
without changing its wave-length. Here we have a 
welling-up of hydrogen and other substances, giving 
lines bright in the spectrum, and Macs constantlj i 



THE A TMOSPHERE OF THE SUN. 



325 



the condensation of vapours, the presence of exhalations, chap. xix. 
and the vast weight of the atmosphere, was utterly right, 
so far as modem science can endorse his opinion. 



I must now come to another part of my subject, slightly 
different from what has occupied our attention hitherto. 
I refer now to the work done in the laboratory and not in 
the observatory ; and I trust you will admit that the future 
study of the physics of the sun will be the combined work 
of the astronomer, the physicist, and the chemist. 

The different lines we see in our instruments when we 
examine the solar prominences are not all alike. Some 
of the lines vary very much from the appearance of the 
C line of hydrogen for instance. In fact, in one line, the 
P line, we get a trumpet-shaped appearance. The line 
widens as it approaches the sun, so that it resembles an 
arrow-head, resting on the thin absorption line which forms 
the shaft. This is not only true of the hydrogen lines, but 
of the lines due to the injection of other substances into 
the chromosphere. 

Now, the question is how to determine experimentally to 
what this widening of the F line is due ? 

After Kirchhoff*s announcement of spectrum analysis, 
^^^ of the great points connected with it was that the 
^P^ctrum for each substance was absolutely distinct, and it 
was generally regarded as invariable, so that — given the 
^P^ctrum — you could make no mistake whatever about 
^^^ element, the vapour or the gas which gave off this 
^P^ctrum. 

We have, however, got further than this now, and we 

^^y say that certain conditions of each substance give us 

^^ain spectra. So that it may be said that spectrum 

'^alysis is competent not only to indicate the substance, 

^^ also somewhat of the physical conditions in which that 

*^^tance is existing at the time it is giving us its spectrum. 

_"l\icker and Hittorff were the first to give us an idea of 



B faring of 
the labora- 
tory on the 
obstn'atory 
work. 



S/cctrum 
analysis 

can indi- 
cate the 

physical 
state of 
substances. 



th 



'^ extension; they held that for certain gases and vapours 



i 



326 SOLAR PHYSICS. 



CHAP. XIX. which they examined there was a spectrum of the first 
' order, a spectrum of the second order, and so on ; but the 

cause of these various spectra was not stated with any 

certainty. 
Changes in I have been fortunate enough to be associated with Dr. 
spectra are Prankland in a continuation of the work of Pliicker and 

not due ^o . . 

tempera- Hittorff; and m connection with the mere astronomical 
/«r<f perse, r^gults to which I have referred, we, as a result of athrtc 
years' inquiry, have come to the conclusion that there is 
a definite reason for certain changes in spectra ; and that 
these changes are not due to temperature per se, but to 
pressure. We find, for instance, that by increasing the 
pressure, say of hydrogen, we thicken out the lines, and 
especially the F line, exactly as it is thickened out in the 
lower region of the chromosphere. 

A Sprengel pump and induction coil enable us easily to 
recognize the wonderful changes in the colour of hydrogen 
as its pressure is varied, and the addition of a small spec- 
troscope shows us the wonderful changes in the thickeue 
ing of the different lines which accompany these changing 
colours. Since by varying the pressure of hydrogen ^wist 
can thus vary the thickness of the lines, it is possible W 
observe the spectrum of hydrogen in a tube, and to pi if 
the spectrum we get from the hydrogen in the sun, side b 
side with the spectrum we obtain from the hydrc^en in tl 
tube. We can vary the pressure of the hydrc^cn in tl 
tube, so that its spectrum exactly fits, so to speak, tl 
spectrum of the hydrogen in the sun ; and hence we ^^ 
enabled to determine the pressure of the hydrogen at th 
sun. And this, no doubt, will some day be done; but tk 
thing is not quite so easy as this. There arc more clem 
than hydrogen in that part of the sun which so co: 
niently gives us these bright lines ; and we have i» * 
questions of pressure not only to take into account ^ 
actual pressure of the hydrogen, but the combined pressor 
so to speak, of all the vapours which exist in that stiato*' 
and so we have a very great inquiry before us befoit* 



THE ATMOSPHERE OF THE SUN. 327 



more approximate estimate of the pressure can be definitely chap. xtx. 

stated than the one we have already given. 

'We can easily establish the extreme tenuity of the Demon- 

atmosphere in the upper regions of the sun's atmosphere, ^j^^J^ 

wliich are visible to us in eclipses, by such an experiment t^fthtHpfir 
as the following one: — I have in this globe some hydrogen, *^^^*P^^- 
extremely attenuated, though perhaps not so attenuated as 
the hydrogen in the confines of the sun*s atmosphere, but 
it so nearly approaches it, that when we examine the light 
produced by the passage of a spark by means of the spec- 
troscope, we see exactly the same hydrogen spectrum that 
eclipse observers tell us they get from the exterior portion 
of the sun's atmosphere. Most of you are under the im- 
pression that hydrogen is a red gas. The prominences of 
the sun are red, and built up of hydrogen ; but the hydrogen 
'n this globe is not red at all, it is green. 

By such methods as these, — by determining first of all 
the conditions which change the lines from thick into thin, 
or thin into thick — then noting the exact thickness of the 
'ines we wish to match — then changing our pressure and 
fitting it as well as may be so as to represent the same 
phenomena artificially, — we introduce a new method of 
inquiry into solar physics of the most tremendous power. 

Let me take another instance. By sealing a piece of 
sodium in a tube with a rare atmosphere of hydrogen, 
^^d heating the sodium so as to fill the tube with layers 
^' sodium vapour, which are denser and denser as we 
approach the source of the vapour, Dr. Frankland and 
"^ysclf have shown that when we allow a beam of light 
^ traverse the tube, the absorption of the sodium vapour The 
^^Pends upon the density we choose to give to it, and that 'll'^Jl.%^{ 
"^ thickness of the absorption line varies with the density J^^patds on 
^* the vapour. In fact, we can thus artificially reproduce ^^^^f^H^^'^ 
. ^ exact thickness observed in the D lines, when they are rafour, 
'Hnest in a high prominence and thickest in a deep spot. 
*^ere again, I think, you will agree that we have abundant 
^^of of the continuity of the .solar atmosphere. 



328 SOLAR PHYSICS. 

CH AP. XIX. The sun, in fact, has not only taught us how to attack 
him in this way, but in these experiments we have fore- 
A quattti' shadowcd a new science altogether, a Quafititative SffC' 
tatwespec^ trum Analysis. 

analysis. A reference to another experiment will make this more 
clear. When the vapour of magnesium is injected into 
the prominences, we get the well-known triple line i, 
showing us three bright lines beyond the sun's limb. But 
the lines arc not of equal height. I explained it in this 
way. While in the case of the F line reduced pressure 
thins the line, in the case of b reduced pressure not only 
thins all the lines, but abstracts one of them from the 
spectrum altogether. To test this explanation Dr. Frank- 
land and myself prepared a tube in which the electrodes 
were composed of magnesium, and we observed the spec- 
trum of the magnesium vapour between the poles while 
the pressure of the hydrogen in the tube was gradually 
reduced. Judge of our delight when we found that after 
a little time the third line began to disappear; and that 
when the pressure was still further reduced, the Hne vanished 
entirely. 

This at once explained what I had seen on the sun, bot 
this was not all. Knowing that both in the chromosphere 
and in our experimental tube the magnesium vapour was 
associated with hydrogen, it was an interesting inquir)*to 
see how the spark would behave when taken in air wlhout 
the intervention of any tube. 

By means of a new method of experimentation in whick 
the spark was treated as the sun, and made to throw i& 
image on the slit, the appearances observed on the »■ 
were reproduced exactly by the spark. The nearly ptf* 
magnesium vapour close to the pole gave us the tb* 
lines, while further away, when the vapour was mixed with 
air, we got the spectrum due to reduced pressure; in Wi 
the reduced quantity of the vapour, so to speak, g*** 
the same effect as the reduced pressure in the oditf 
experiment. 



THE ATMOSPHERE OF THE SUN. 



329 



Expert- 
matts. 



t is easy to show also that with increased pressure chap.xix. 
les such an increased brilliancy as we get in the sun, 
ire going inwards we have first the faint corona, then 

hydrogen layers visible only in an eclipse, then those 
«rved every day by the new method, and last of all the 
3tosphere itself. 

Sere is a simple experiment bearing on this point : I 
/e in this tube some hydrogen at a very low pressure, 
1 at the bottom of it some mercury. So long as the 
rcury is cool the spark passes through nearly pure 
irogen, and the tube is lighted up with only a faint 
cnmer, the equivalent of the auroral discharge. But if, 
tead of having our mercury vapour almost absent from 
' tube, in consequence of the low temperature of the 
aid supply at bottom, we drive this liquid mercury at 
' bottom of the tube into a state of vapour, we find that 

not only change the colour of the discharge in the lower 
ions of the tube, but imitate what we get on the sun 
^If. As the discharge passes through the denser layers, 
1 renders them incandescent as they are at the sun, the 
Uiancy increases enormously as the source of the vapour 
Approached, and in each stratum of different density 
the mixed gas and vapour in the tube we have the same 
rease of brightness as we observe in the similar strata 
the sun's atmosphere. 



\lthough the laboratory work to which I have alluded 
lecessarily of such a kind that the fact that the changes 
ivhich I have referred are due to pressure and not to 
iperature may be considered by some to be still open, 
tlink that a phenomenon of not unfrequent occurrence 
ich presents to us a bright line prominence on the sun's 
:^ flitting high over a spot, the bright lines of sodium 
Ae prominence being thin, and the dark lines in the spot 
tioderately thick, may be accepted as final, for we must 
ume that the prominence is hotter than the spot which lies 
^ond it, or otherwise its lines should not be bright, and 



Promt' 

ncttces 07'er 

spots. 



_f J fsrsjcs. 



. this bemg xy. it i 
3oJeiv vbicQ f<»M'fc* 



If, ihea, we g« t 
jf the iines. ifae inc 



^PiiD^ aantefy^, the thidtenix^ 
i WBMiy of tbe diftmt b^crs 



aaii oC the liioemit •tt.zicuts, T afMmy oh amul botfa herc- 

anc <3a the 5uii ; ii '^ : ^i;£ jH ibcK ckaBgcs repRKbicMi 
in our Ubcnt. :r:<^. ii i cafl get at die ^ui9es of thms. 
(C is not M-: -■« cfaoC if sadb mA be asoda- 

Gusly and : : i^rHtrd OS; in coone of tinK v^f 

jhail irrS--, -; ;'ZiUy,Jt ft profboad tos^rt inl«=» 

the TLa:.ire ot =oUr paenoOMMk wlale at the same tinK= 
the i-^eczr j<-zope bos added ao eao t mo us oetr fidd i»' 
oiner.-:i.ti>-ja la the p-.'sfibiK^ it bas aAmled us of 4!ail9>^ 
dirrioiclhs^ th : Z'- -..-^ : n^ ^aoaaiaas, »ad aatetuls of ifa^ 
prominca^ei- -^ni::-. ari at cmce the most beantUul aa«3 
most delicate i-iications of the forces at work in (w *" 
central luntioan-. And I imd not stop to poult out tha' 
our centra! luminan.- is not alooe in question. All ih* 
kn.mlcd;^e «-e can ever hope to g^ of the physia' 
cu(utituli-'n of those distant orbs, which illuininate wh>t 
art to us the distant reahns of space, mast be got b>' > 
Uudy of iolar physics. The beautiful researches of Cu- 
rington. De !a Rue. Stewart, and Locwy. hax'e establiiiicJ 
that our sun is a v.iriable star. Among the work lo be 




THE ATMOSPHERE OF THE SUN. 331 

:er such a work as this ? Our Government, although chap.xix. 
)tto is ex luce lucellum^ will probably decline, though 
certain it is a work which other Governments will 
lip; we are driven then to our great corporate 
;, and among them especially to our ancient 
rsities, as almost our only hope in our own land, 
the time comes when the true and best duties of 
emment are known. 

1 why should not Cambridge take up the work which 
mguishes for want of help ? Surely, in doing so, she 
only be following up her ancient precedents, would 
entire sympathy with her past. 

> not dare to make a more urgent appeal, but I do 
•e to express a hope, that when some one, perhaps in 
ng future, and in this Senate House, tells a succeed- 
eneration how the many secrets of the sun have 
•ead, he shall be able to refer to a physical observa- 
t Cambridge, as I to-day have referred to Newton's 



THE ENGLISH ECLIPSE EXPEDITION, 1871. 



I. — Preliminary. 

CHAP. XX. As will be seen from the accompanying map, the central 
line of this eclipse of the sun in 1871 first met the 

Regons earth's surface in the Arabian Sea, and, entering on the 
^eclipse Western coast of Hindustan, passed right across one of the 

wastotaL most important parts of that country in a S.E. by E. direc- 
tion. In this part of the peninsula, the sun was about 20* 
above the horizon when totally obscured. The duratiot 
of tota ity was two minutes and a quarter, and the breaddi 
of the shadow about seventy miles. On leaving the 
eastern coast of the Madras Presidency, the central line 
crossed Palk's Straits, passing about ten miles S.W. of the 
island Jaffnapatan, and over the northern part of Ce}'lofl, 
where the small towns of Moeletivoe and Kokeley lay 
near the central line, and the well-known naval statiofl 
of Trincomalee, about fifteen miles to the S.W. Coo- ■ 
tinuing its course over the Bay of Bengal, the shado* 
crossed the S.E. point of Sumatra, and touched the south- 
western coast of Java, where Batavia, the capital, ws* 
nearly sixty miles N.E. of the central line ; and twoodiff 
smaller towns, Chidamur and Nagara, were also vcfj 
near the middle of the shadow path. In the Admiialtf 
Gulf, on the N.W. coast of Australia, the eclipsed sof 
was only ten degrees past the meridian, and not fax fro« 
the zenith : in consequence of which the totality lasted 



SOLAR PHYSICS. 



■ four minutes eighteen seconds, or only four seconds L^^ 
than the time of greatest duration. Lastly, pass/fl„i 
through the most barren and uninhabited portion of 
Australia, and crossing the Gulf of Carpentaria and the 
York Peninsula, the shadow ultimately left the eanJi's 
surface In the Pacific Ocean. 

At the outset not too much was known about the chance* 
of weather at any place ; but what was known seemed t«3 
point tn a fair chance of success in botli India and Ceylor*, 
as the eclipse occurred during the monsoon, but in my 
case the experience of the last expedition showed that 
for such a momentary phenomenon these chances nee-*^ 
not be taken too seriously into consideration, seeing iha-t 
then where the finest weather was predicted a terribl* 
pall of cloud covered the sky. 

Uefore 1 proceed to give the results of this expeditioT*- 
it is necessary that I should state as briefly as posibl* 
how it came to be undertaken, and the plan of opcralioi»s 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 335 



" In my opinion the fundamental points of attack are : chap, xx. 
" a. Spectroscopic observations made with such an 
instrument as the one I took out to Sicily, eqtuitorially idur to 
mounted^ and with reference spectra. '^ ^^^ 

" )9. Photographic observations made with such an 

instrument as the one I took out to Sicily ; namely, a 

camera with large aperture and small focal length, 

eqjiatorially mounted, 

" Perhaps I may clear the ground by stating what, in 

ly opinion, is comparatively UNIMPORTANT, so far as the 

rucial points are concerned, though to be tolerated if the 

rucial points are strongly taken up. 

" a. Photographing prominences. 
")9. Sketching anything but the changes in the 
corona. 

" 7. Polariscopic observations. 
"8. Observing Rally's Beads. 
"There should be one instrument, and Mr. Pogson could 
>robably provide this in India, to determine the position 
>f prominences before and after totality. During totality 
^y slwuld fwt be observed at all except incidetitally, 

"At each place {i.e, India, Ceylon, Australia) the spec- 
roscopes should be employed for half an hour (to be on 
^c safe side) before totality, in scrutinizing the crescent 
^ >ts narrowest place and the chromosphere outside the 
^Mowing limb of the moon. 
" At each place, as before defined, there should be a 
^^ctroscope with a finder, and equatorial motion (or some 
luivalent arrangement) directed to the sun's centre, to 
^^^rd any changes which take place in the spectrum from, 
^y. half an hour before to half an hour after totality, 
*^d during totality, bien entendu. The relative darkness 
^'' brightness of the lines should be recorded every ten 
*^^nds. 

'This spectroscope should have moderate dispersion, 
Ui^e object-glasses for collimator and telescope, and with 
*^1 length such that two or three degrees round the sun 



336 



SOLAR PHYSICS. 



CHAP. XX. should be taken in (i,e. i** or I J"* from the sun's centre), 
and a large field. 

"To come to the details of the expedition to Ceylon; 
I am of opinion that it need not exceed the following 
numbers, as my Sicilian experience has taught me that 
we may depend upon much valuable help from the officen 
at the place of observation : — 

" I Telescope-Spectroscopic observer ; 2 assistants 
" I Photographer ; 2 assistants. This duty pcrhap 
may be entrusted to skilled Sappers. 

" I Spectroscopic observer ; i assistant, or 8 i» 
all. 
" Among general observations, I would point out as 
being of extreme importance : — 

" a. Rays before, during, and after totality— thdf 
length, direction, and colour. 

"/8. Colours of the various layers of chromosphere^ 
and of clouds and landscape. The order of these 
colours is of great importance. 

" 7. Dark rays or rifts ; whether they change, and 
whether they extend to the dark moon, or stop short 
above the denser layers of the chromosphere. 

"S. The colours of the corona between bright* 
dark rays. 

" e. All changes in corona. 
" f. Comparative brightness of rays and chroBK)' 
sphere and outer corona." 



Action of 

Royal 

Society, 



The Council of the Royal Society at once took enaptic 
action ; instruments were sent off to Australia, and il 
promised well, when owing to events which I need fA 
further particularize the efforts of the two learned boditf 
entirely collapsed. Public attention, however, had beel 
called to the importance of an expedition, and thcwitfc* 
drawal of the Astronomical Society from all paitidp** 
tion exercised no prejudicial influence whatever, ft* ' 
the meeting of the British Association at Edinburgh t^ 



THE ENGUSH ECUPSE EXPEDITION, 1871. 337 

President, Sir William Thomson, in his opening address, chap. xx. 
liter referring to the recent sun work,, went on to say : — " 

•• During six or eight precious minutes of time, spectro- Extract 
scopes have been applied to the solar atmosphere and to sir^ 
the corona seen round the dark disc of the moon eclipsing Thomson's 
the sun. Some of the wonderful results of such obser- ^*^^^^' 
vations, made in India on the occasion of the eclipse 
of August 1868, were described by Professor Stokes in a 
previous address. Valuable results have, through the 
Hberal assistance given by the British and American 
Governments, been obtained also from the total eclipse of 
last December, notwithstanding a generally unfavourable 
condition of weather. It seems to have been proved that 
at least some sensible part of the light of the * corona * 
*s a terrestrial atmospheric halo or dispersive reflection of 
*ke glowing hydrogen and 'helium* round the sun. I 
*^lieve I may say on the present occasion, wlun prepara- 
'^ must again be made to utilize a Total Eclipse of the sun, 
^^tthe British Association confidently trusts to our Govern- 
^^^^Ht exercising the same wise liberality as heretofore in the 
^^ests of science'' 

* his expression of opinion was at once followed up : 
^ ^ general meeting of the Association it was resolved to 
J?*^ for a sum of 2,000/. in aid of the proposed expedition. 
*^is was granted the day after the application was made. 

^ committee was at once appointed. Instructions were Work 
*J^pared and a communication addressed to the Directors '^ f'''"" 

the Peninsular and Oriental Steam Navigation Com- 
^^y, who entered so warmly into the scheme that the 
^Uced terms they offered amounted really to a mag- • 
^cent private endowment of so liberal a nature that the 
"^Oimittee resolved that the Expedition should consist 
' twelve persons. Invitations were accordingly sent to 
^*^5sen, Young, Angstrom, Zollner, Respighi, and Peters, 
J^Ong foreign men of science, and to a large number in 
*^^ three kingdoms who were known to be interested in 
^Ur or spectroscopic research, begging them to take part 

z 



HE ENGLISH ECLIPSE EXPEDITION, 1871 

(continued^ 

II. — The Bekul Party. 

iTUNATELY for the Expedition, General Selby, who chap.xxi. 
2s great interest in science, who is an admirable artist, 

who had observed the Eclipse of 1868, had already 
►Avn the whole weight of his position into the arrange- 
its, and sent tents to Bekul. He now offered guards 
the various parties, stated his intention to issue an 
ir calling for volunteers among the officers at Canna- 
- and Mangalore, and expressed his intention of coming 
iekul himself to take part in the work. The light- 
rtedness, therefore, with which the remaining four of the 
;>edition — Dr. Thomson, Captain Maclear, Mr. Davis, 

myself — saw the anchor come out of the turbid water 
' steam got up for the last final run' into the jungle, 
y be imagined. Bekul had been a mystery ; nobody Bekul, 
-w anything about Bekul; the word was printed re- 
tkably small on most maps : what was it like } Trinco- 
lee was a naval station ; Jaffna a large and important 
'^n ; Poodocottah the residence of a Rajah ; Manan- 
Idy the head-quarters of all kinds of new industries, 

We were in the flag ship, H.M.S. Glasgow^ to the captain of 
ch— Captain Jones — ^the Expedition can never express sufficient 
utude. The final run here referred to was from Cannanore. 

Z 2 



340 SOLAR PHYSICS. 

CHAP. XXI. chinchona, coffee, teak, and the like, with any number of 
English, and a club to boot : but Bekul ! We should at 
all events be looked after. 

No wonder the good ship Glasgow would have steamed 
right past the place if Mr. Lewis M'lvor, the indefatigable 
Assistant Collector, and Mr. Pringle, the representative of 
the Public Works Department, who had been sent to meet 
us, had not been on the look-out, and come off in a small 
boat, waving a large flag, so energetically that the ship's 
head was turned slightly towards the shore, and then, 
as the men in the chains gradually got their song down 
to " Quarter less six," we saw a ruined fort, with crowds of 
natives, and a solitary house among the cocoa-nut trees. 

This was Bekul. The natives were not natives of the 
place — for, practically speaking, there was no place— but 
men who had come many miles to see the great fireship 
that was to bring the astrologers, and they were nov 
feasting their great eyes with the unaccustomed sight 
There was more excitement for them to follow. When 
Mr. M^Ivor had made all arrangements for landing ifl 
the morning, he went ashore, and, as a measure of pre- 
caution, ordered the fire which had been lit to marklw 

Impromptu landing-place to be kept in. In doing: this the sun- 
Hon^' scorched grass in the vicinity was soon ablaze, aw 
looked so like an illumination in honour of the ship* 
arrival that a blue light was burnt, a rocket sent ^ 
and a gun fired, to the intense delight of the nativts* 
who fully appreciated the impromptu illumination. 

In the morning, on landing, which was a most ticklii 
operation in the surf, work began in earnest with the ri$W 
sun. The spots chosen by Mr. M'lvor for the tcmp*^ 
rary observatories for the telescopes — in old Tippoo's f"*^ 
which commanded the whole horizon, and a vacant sp** 
near for the photographic work — were inspected *■" 
found so satisfactory that the instruments immediiW 
on landing were taken there, unpacked, and before W 
were approximately in their positions. While this^ 



'J 



THE ENGUSH ECLIPSE EXPEDITION, 1871. 



341 



w\g on the Glasgow went to prize firing, an opera- chap. xxi. 

which had been delayed while the Expedition were 

l)oard, for fear of damaging instruments and break- 

irig^ object-glasses. This tremendous proceeding on the 

pa.irt of the ship, and the wonderful similitude of the 

tel^55copes to the native idea of a big gun, soon wrought 

3. '^^.^onderful change in the ideas of the dwellers along Alarm of 

the coast. The Eclipse was a pretence. There was *''/«'«• 

I If otherwise, why the firing with shot } Why 

py the fort } Why erect big guns in the most com- 

nianding place in its, to them, vast extent ? Why these 

soldiers from Cannanore.^ Instant action! All high- 

^^^^t^ women and all gold into the interior ; men still to 

^^tch the action of the " gods," and if possible probe their 

"^otivesand intentions to the very bottom. Of course the 

^^Jy way of meeting this rumour, which might have proved 

^^O'' unfortunate to us, was to allow everybody free access 

^^ ^he observatories. When the natives found that the 

^ guns were made of very thin metal, and that the 

•^^Sgest of them when looked boldly into showed merely 

^^^ face of the inspector considerably enlarged, all fear 

P^^sed away, and probably the women were recalled, but 

^*^ this point I have no certain knowledge. 

After the inspection of the fort the party marched down Tht 
^ the encampment about half a mile to the north and along ^"^(<^ 
/^^ shore in a shady nook. In the centre was the little surround- 

^ngalow long disused, but now done up for the occasion ; "'^' 
^^Und it — leaving, however, the sea-view open — were ar- 
'"^nged the tents. The Collector of South Canara (Mr. 
A. M'C. Webster) and his able assistant (Mr. M*Ivor) had 
^ade every possible arrangement; and when I say that 
this included necessarily the bringing of our bread some 
forty miles every other day or so, some idea of what had 
to be done may be gathered. Few seemed to know the 
capabilities of the place in the local production of those 
articles, such as cheetahs, snakes, &c., for which India is 
so celebrated. Trying to sleep, therefore, in a hammock 



I ;=-_ ra tiu: iira^aijow, ■; 



• tiC" ~:TirtOT ! 




■ 6w the hega w er. M 
» lewl of the jacbl'ie 
: soocpaDSS are looktd 
ipi.c i^ mi^^i zat^mKOtZ aHl aooL As it was. oolf cm 
i^i^^;jzzt. cce jTAfca; aad flMe soocpioa made their ^)pcar- 
a3<i± v^Ue Tf vere ^ Bdari. TW fiet and secood gnce- 
nZlv -v^iscr^-v. :bc Imect «as sifeir honied. 

Le: tne s^^: r sajawofd aboBCOBrwock En tbccam^. All 
Tu^e £- 4.3c r tboE^Oitf^ Aeb lea and a walk to the 
rjxer:-3iaries to mik licibvc Ae beat of the day begin. 
By :u=€ we ^7. had had qate eaooEh. and, indeed, some of 
:be party j-X; s'^rrer ftar a day^ or so bjr exceeding this limit 
The^ h jzne, a bath and br^c&st. a rest and thi» tifin- 
A'z^r :hr:^. obsenratoo- a^aic friMD aboot four till eight (w 
Thea back to the camp for die 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 343 



on the memorable day, however, it was found that the ciiap.xxi. 
clouds had all disappeared during the night, with the 
exception of a low bank to seaward which did not alarm 
us ; so the whole party repaired to the observatories in 
the best of spirits. 

I must content myself with describing what went on in The 
the fort. Imagine a round, rough, embattled platform, ^t^'J"^ 
some thirty or thirty-five feet in diameter, with two large 
telescopes placed nearly in the middle, pointing to the 
rising sun. At one of them, a large 9I reflector, was 
stationed myself; at the other Captain Maclear and Mr. 
Prii^le. Close to the reflector is a table with some in- 
struments upon it, at which are sitting two persons — one 
Captain Bailey, who is to tell how the time is going on ; 
the other a native employed in the Collector's office, to 
take down anything that is said, his paper being carefully 
marked, so that the place of his record denotes the time 
of the observation. By the table is standing Mr. M'lvor, 
whose duty it was to assist me in opening the slit of the 
spectroscope, if that should be required, and the like. 
Mr. Fernandez is there, too, to watch the clock, and dis- 
connect the telescope from it at the right moment. Captain 
Christie is acting as amanuensis for Captain Maclear. In 
the other corner, at tables, arc General Selby, Colonel 
Farewell, and Judge Walhouse, with cards and telescopes 
before them, ready to sketch the corona. This is the 
attacking party, and there arc police to keep out intruders. 

As the first contact took place at Bckul a few minutes f'i*'st 
. after sunrise, when the image of the sun was unsteady, 
the exact time could not be noted, but that was a small 

I matter. Slowly the eclipse crept on ; swarms of small 
Rajahs* squires, and natives of all sorts and conditions, 
rafridly coming up in their manchicls, and taking up their 
position round the fort, which they are not permitted to 
enter. 

There is strict silence in the fort, and the work of re- -AV//,,. 
rording the comparatively unimportant phenomena visible 



rend the air as the monster seems to thei^ 
upper hand ; the excitement increases, and evid( 
thing is afoot. Mr. M'lvor's sharp eye detected : 
sacrifice of fire, the intended fuel being the lor 
grass covering the landscape exactly between th 
the eclipsed sun. In a moment he pointed thisc 
Christie ; in a few more a posse of police wa; 
out the flames, and the smoke-bank, which thr 
upset all the work, gradually died away ; the m 
ever, still continued, and Rahoo worked its wick 
It is now time to return to the fort. Capta 
announces bright lines in abundance. 1 had i 
to observe these, whisper a word or two, and 
large spectroscope, before I exclaimed "Stci 
signal agreed on for commencing the countiq 
Instantly everybody in the fort heard Captai 
clear voice ringing out, "You have 120 seconds," 
in the leaden-coloured, utterly cloudless sky, sho 
eclipsed sun ! a worthy sight for gods and mc 
rigid in the heavens, was what struck everybody 
ration, one that Emperors might fight for; a 
times more brilliant even than the Star of India, 
then were! a picture of surpassing loveliness, a 
one the idea of serenity among alt the artist) 
going on below; shining with a sheen as of silve 
built lip of rays almost symmetrically arrang 
a bright ring above and below, with a markei 



THE ENGLISH ECUPSE EXPEDITION, 1871. 



34S 






m, 1474 longer than F." Following close upon 
Mr. M'lvor's command, " Polariscope/' we got the words 
** Volarization vertical over everything ; strong : " this was the 
verdict of the Savart. Next the biquartz came into play. 
"Yellow and brown, with green on both sides, faintly 
radial/' was next heard. Then from Captain Bailey, "You 
have eighty seconds more." This was the signal for ob- 
serving the eclipsed sun through a long train of prisms, 
an attack in which I placed great confidence, and which 
I then rushed to make. " Four circles, 1474 same size [as 
the rest] ^ and faint," was at once written down by the 
amanuensis. Then another manoeuvre. "You have still 
thirty seconds more," said Captain Bailey. In a moment 
Captain Maclear and myself changed instruments — I to 
observe the structure of the corona with the Astronomer- 
Royal's 6-inch telescope, Captain Maclear to note the 
spectrum of any part which I might feed him with, in 
a spectroscope of large dispersion mounted on my own 
i^efractor of slightly greater aperture. 

" Definite structure five minutes from sun," *' No spec- 
trum," "Structure like [that of the great nebula in] 
Orion," *'No spectrum," were now uttered antiphonically 
oy myself and Captain Maclear in rapid succession, and 
^^^ eclipse was soon over to the outsiders, apparently 
Wore its time ; but not to me at the refractor. *' Structure 
^^ili visible," " Still visible," " Still visible," now went on 
lOr nearly three minutes, and then the corona vanished 
'"to tihinair. 

^^ the fort Captain Maclear, and below, Dr. Thomson 
and ;^j. Davis, did noble work ; and far away at Jaffna, 
^'^^^omalee, Poodocottah, and Manantoddy, were others 
^11 'vvorking out the programme; while at Ootacamund 
^° -Avenashi were Janssen, Tennant, Herschel, Pogson, 
neanessy, and a host of others, strung up to the same 
point: of tension. Of the results of the work of course 
^s Vet I can say nothing, except that at Bekul Dr. 



CHAP. XXI. 

• Prism 
train and 
polariscop^. 



Structure 
of corona^ 



Other 
observers. 



'I h» 



3ve bracketed the words omitted for brevity's sake at the time. 



from a slight attack of fever — the sure result 
here — were being rapidly carried in maiichie 
backs of a host of bearers, to gain intelligence ol 
parties and to confer with Jansseii, Tennant, an 
Little rest was there to be got along the jungle i 
with the plaintive, rhythmic moaning of the bi 
flashing of the torches in the dark night, the c 
innumerable fords, and, above ali, the great 
"What have the other parties done?" still unsoK 

This refers lo the fact thai the pholographic arrai 
Bekul, which formed one of the fundasicntal pans of the 
done, were conducted partly at Lord Lindsay's expense by 
Mr. Davis, who formed part of the British Association E» 




i ENGLISH ECLIPSE EXPEDITION, 1871 

{continued), 

III. — A Letter from Ootacamund. 



CH. XXII. 



Surely if eclipse expeditions had their mottoes, that 

^* the expedition of this year should be per mare per 

^^^Tam ; for it has been per mare per terram in our case 

^ith a vengeance ! Probably when we return, the curious 

^Individuals who total up in the Times the aggregate number 

^* years those people have lived whose deaths are there 

'^^corded, will, in asking us for our autographs, beg also a 

^^tailed statement of the number of miles each of us 

"^s travelled in the performance of our duty. I fear it 

^''11 be very difficult to give the information ; and if the 

^^'Hperature in the shade be wanted too, the thing will be 

Perfectly hopeless : for, thank goodness, we took the pre- 

^^tion to bring no thermometers ; had we done so and 

ioolced at them, it might have been all over with us. Let 

^^ point my remarks. A week ago I was at Bekul, having 

ravelled I know not how many thousand miles by sea, and 

. ^^ing scarcely set foot on land for a month. We wTre 

^he jungle, the heat was burning, some of us had fever, 

fk* ^* ^^^ opium which enabled me at all events to get 

, '"^Ugh the day of the eclipse, for it was that memorable 

^y^ just a week ago. Since then, by night and by day, hidiau 

*"- Thomson, Captain Maclear, and myself have been — travelhni. 




sa^ja jvrsrcs. 

L jf/teJ a too stroi^ 
I ricficulously Ucking 
' rnm Bdcal, niw 
r bearers with tbeir 
^ keeptBg ibp. 
i labour they nw 
r as cf a ootain journey wbicb 
tern «■ aes's shoulders : now io 
n hour, thsnki U> 
I has upset my plus 
r canines, average speed 
-^1 r^cs 12 '^ya. kcHpaaKwc of on-iagc at aooo un— 
o> wr aai L±£^ ia tk hooc Uutsit of the Madras Car^ 
-r=c C-:wBriirT Ok &tf thcH- caiiiagcs were as good is 
t>^r irrxs^i-rxTts ami tkc Speed of their hoRes ! And 
■s;-"- be^c I i= ■hiiLiTag. JMiUMuJid by hoar- frost, with 
i ^:-i^"r« ::" i dificmlty of breatinng in this higher iir 
eot the jungles, but all ttieuinc 
I beeves of roses, ahhough it »* 




ENGUSH ECLIPSE EXPEDITION, 1871. 349 

Janssen and Colonel Tennant have had the ch. xxii. 
tesy t6 send me, that they too saw the eclipse 
lo did Mr. Pogson, as I gather from the news- 
it of course the details of their observations are 
wn to me. Hence, I can only give the facts 
>y the party at Bekul and Poodocottah ; Prof. 
nrho observed at that station, having joined me 
>re, the station on the Madras Railway, at the 
2 hills which we ascended yesterday from 4.30 
P.M. 

)re I say a word about the observations them- 
5 incumbent upon me to express our deep obli- 
the Supreme, Madras, and Ceylon Governments 
ignificent manner in which they have aided us. i^obUaid. 
ould be more complete than the arrangements at 
de by the Collector, Mr. Webster, and his assist- 
Iclvor, both for the work to be done and the 
f those who had to do it. The same must be 
le Poodocottah party, where not only the Collec- 
/hiteside, but the Rajah did everything in their 
I latter loading the observers with presents when 

Wc have at present heard only of the discom- 
e Manantoddy party, and it is clear that here the 
ngements were in strong contrast to those else- 
'he Ceylon parties, who parted from the main 
Jalle, have doubtless been well looked after: as 
•"yers, the Surveyor-General of the island, accom- 
d aided them in their observations, 
rings us to another part of the arrangements. 
on party had the unreserved use of the Govern- 
amer the Serendib, to take them from Galle to 
:es of observation, Jaffna and Trincomalee, both 
ast, and the accommodation onboard was perfect. 
an parties proceeded to their various destinations, 
rts on the coast nearest to them, in the Admiral's 
the Glasgow, which, however, could not remain to 
m back — a circumstance which has given rise to 



ptat risk fof the 

all along the Vac, 

tfce cout to Bombay or 

This of course 
hope foe the btst. 
their utmofl in 
wi hawrfing them over in 
GtyvcnineDt officers t^o 
Sti^ aUo^ it was doI to be ^ 
•f the ship hu been the unfor- 
it de »rr»trjB«rt!f> Xothii^ could 
•f Ae Afbinl, wbo vacated hu osn 
: •■■■; «f OfitaiD Joacs, who Kick 
and hdped the 
; aad of Ae officers of the ship 
of Mr. WeiMlcr 
's cabcn mto the jiu^te 





THE ENGLISH ECLIPSE EXPEDITIOX. 1871. 3S1 



(ions. 



Colonel Farewell, Judge Walhouse, and others, in sketch- ch. xxii. 
\ti^ the corona. At all stations, of course, most precious 
help in various ways was given by all present who volun- 
teered for the various duties, though some of them lost a 
^ht of the eclipse in consequence. Among those who 
helped in this way at Bekul were Mr. M*Ivor, Mr. Pringle, 
Captain Bailey, who timed the ech'pse, Mr. Cherr}% and 
Capt Christie, the Superintendent of Police, whose presence 
there turned out to be of the most serious value ; for the 
natives, seeing in the eclipse the great monster Rahoo 
devouring one of their most sacred divinities, not only 
howled and moaned in the most tremendous manner, but 
set fire to the grass between our telescopes and the sun, to 
propitiate the representative of the infernal gods. Captain 
Christie with his posse of police stopped this sacrifice at 
the right moment, and no harm was done. 

Now for the observations. Perhaps I may be permitted Detail oj 
to begin with my own, as at the present moment I know 
"^ost about them. I determined to limit my spectroscopic 
observations to the spectrum of a streamer and to Youngs 
stratum, thereby liberating a number of seconds which 
^ould enable me to determine the structure of the un- 
doubted corona with a large refractor ; to observe the whole 
phenomena with the naked eye, and through a train of 
prisms with neither telescope nor collimator ; and finally 
with a Savart and biquartz. I found the 120 seconds gave 
me ample time for all this, but owing to a defect in the 
counterpoising of my large reflector, which disturbed the 
rate of my clock, I missed the observation of the bright 
Imt stratum (assuming its existence) at the first contact. 
At the last contact Mr. Pringle watched for it and saw 
some lines. 

Having missed this, I next took my look at the corona. 
It was as beautiful as it is possible to imagine anything to 
be. Strangely weird and unearthly did it look — that strange 
sign in the heavens ! What impressed me most about it, in 
my momentary glance, was its serenity. I don't know why 



earth, no yellow clouds, no seas of blood — the gi 
Ocean almost bathed our feet — no death shadow 
faces of men. The whole eclipse was centred ia 
and there it was, of the purest silverj- whitem 
not want to see the prominences then, and I d 
them. I saw nothing but the star-like decoratic 
rays arranged almost symmetrically, three above 
below two dark spaces or rifts at the extremities 
zontal diameter. The rays were built up of in 
bright lines of different lengths, with more or 
spaces between them. Near the sun this stn 
lost in the brightness of the central ring. 

But from this exquisite sight I was compel! 
myself after a second's gazing. I next tried tht 
of a streamer above the point at which the sur 
appeared. I got a vivid hydrogen spectrum, 
(i assume the place of this line from prior ob; 
slightly extended beyond it, but very faint thro 
length compared with what I had anticipated, an 
ing downwards, like F. I was. however, astonist 
vividness of the C line, and of the continuous 
for there was no prominence on the slit, I was a 
habitat. The spectrum was undoubtedly the sf 
glowing gas. 

I next went to the polariscope, for which ins 
had got Mr. Becker to make me a very ttme-sa 
trivance- — a double eye-piece to a small teles 
containing a Savart and the other a biauartz 



THE ENGUSH ECLIPSE EXPEDITIOX, 1871. 353 



I. I next tried the biquartz. In this I saw wedges, ch. xxii. 
;tly coloured here and there ; a yellowish one here, a 
wnish one there, mth one of green on each side the 
ction, are all the colours I recollect. Then to the new 
ick — the simple train of prisms which Professor 
ang had thought of as well as myself ; its principle 
ig that, in the case of particular rays given out by 
li a thing as the chromosphere, or the sodium \*apour 
a candle, we shall get images of the thing itself 
ited in that part of the spectrum which the ray in- 
its, so to speak ; we shall see an image for each ray, 
if the prisms were not there. Wliat I saw was four 
uisite rings, with projections where the prominences 
e. In brightness, C came first, then F, then G, and last 
11 1474 ! Further, the rings were nearly all the same 
kness^ certainly not more than 2' high, and they were 
nveloped in a line of impure continuous spectrum, 
then returned to the finder of my telescope, a 3I inch, 
studied the structure of the corona and prominences, 
of the five prominences was admirably placed in the 
die of the field, and I inspected it well I was not 
' charmed with what I saw, but delighted to find that 
i>pen-slit method is quite competent to show us promi- 
xs well without any eclipse. I felt as if I knew the 
g before me well — had hundreds of times seen its exact 
valent as well in London — ^and went on to the structure 
he corona. Scarcely had I done so, however, when 
lignal was given at which it had been arranged that I 
to do this in the 6-inch Greenwich refractor. In this 
ument, to which I rushed, for Captain Bailey had 
told us that we had ** stiU 30 seconds more ** — which I 
d mentally, though not with my ears, as ''only 30 
nds more " — the structure of the corona was simply 
lisite and strongly developed. I at once exclaimed, 
e Orion ! *' Thousands of interlacing filaments vary- 
in intensity were visible, in fact I saw an extension 
he prominence-structure in cooler material. This 

A A 



to the sun, and even S' or 6' away from tl 
was iiotiiing like a ray, or any trace of any ra 
whatever to be seen. While these obsei 
going on, the eclipse terminated for the other 
me. For nearly three minutes did the cort 
impress itself on my retina, until at last it f 
tile rapidly increasing sunlight. I then ret 
Savart, and saw exactly what I had seenduni 
the vertical lines were still visible ! 

Captain Maclear's well-known skill was am 
I need only here therefore refer to the cxti 
his observations, adding what I should have 
that I saw the bright lines at the cusps, as he 
as to draw my attention to them. 1 am,< 
prepared to say that they were visible M 
arc of retreating cusp. 1 

Dr. Thom,son confined his observations \ 
scope, using the Savart, He states that hi; 
were identical with my own. 

Mr. Davis's photographic tent was below i 
which our telescopes had been erected ; am 
after the observations I have recorded wen 
down to see what success had attended his i 
hailed when half-way there with the cheerii 
'■ five fine photographs," and so they arc, tho 
beginning and end of the eclipse being wond 
with, I fancy, slight changes here and then 
point I speak- with all reserve until they i 



THE ENGLISH ECLIPSE EXPEDITION, 1871. 355 

s exhausts the principal work done by the Bekul chap. 
with the exception of the sketchers with General .' 

at their head, who have recorded most marked 
es in the form of the outer corona, and Mr. Webster, 
^as so good as to photograph the eclipse from a fort 
eight miles away, with an ordinary camera, and 
led capital results. 

ct a word about the Poodocottah, the other fortunate Poodocot- 
1 party. Professor Respighi has promised to send 
jults to you with this. About Mr. Holiday's labours 
yfi nothing, except that he has obtained three 
les. 

iceming the Ceylon parties I give you a verbatim Ceylon, 
:t from the telegrams. From Jaffna : " Exceedingly 
; radial polarisation, 35' above the prominences; 

I undoubtedly solar to that height, and very 
>ly to height of 50'." From Trincomalee Mr. 
ey informs me that he carefully watched for Young's 

line stratum, and did not see it, and that 1474 
>served higher than the other lines. 
s is the sum total of the information which has at 
t reached me. It is clear there are discordances 

II as agreements, the former being undoubtedly as 
Jc as the latter. It remains now to obtain par- 
8 of all the observations of all the parties, before a 
ccqunt can be rendered of the eclipsed sun of 1871. 
>f course, will be a work of months ; but if all goes 

trust to obtain information shortly of the outlines 
work done by the Indian observers and M. Janssen, 
Lin now remaining in India for that purpose. In 
leantime I hope the good people at home will 
ve have done our duty, and that all the members of 
^vcmment Eclipse Expedition of 1871 will soon be 
with them to give an account of their work. 

nmundj Dec. 19, 1871. 

A A 2 



I 



iss EOJPSS £Jcr£jy/TWX tijt 



I -nm. Othex PAr 

I zr-.c~sKd ±aoekr «A^ 1 «as ■ p o MCBtio o of more 
:r^.r-7-.arf>rc sj to Ac woik doH^ aot oolf by t&c Britidr 
-\^.odr>:c rorties, bit hf dnsc fcpccsaitii^ the Indiu 
i^- Fr:^d: '^-iifdaiBEBfes. Let aic bow cndcawofW 
T'^i'tr-. TT.y rro^aise;. aamg that smce that commom- 
car ■- -3-^ r-Tr::;-ai I Itavg had the happiness of hearing 




THE ENGLISH ECUPSE EXPEDITIOX, 187 1. 357 



Yothonore with those taken by Mr. Davis. Unfortu- chap. 
^tely, as has been already stated, we missed each other, ^"^"'' 
^i^d so an absolute comparison of photographs did not 
^^ke place ; bqt from the drawing it was evident that in 
the two series the main form of the corona was the same, 
^he photographs I learned were very sharp and good, and 
<^e appreciates their value the more when it is known that 
^ly a very little time before they were taken, any success, 
^V'en a partial one, seemed out of the question, so per- 
sistently did cloud and mist hang over Dodabet on the 
cv-entful morning. I gathered that the spectroscopic 
observations had also been successful, and that a con- 
tinuous spectrum with 1474 had been observed. If more 
lines than this were not seen, it is easily to be accounted 
for by the relatively long focal length of the object-glass 
employed to throw an image of the eclipsed sun on the 
slit. 

Not until the morning after my interview vnth Captain 
Waterhouse did I learn the whereabouts of Dr. Janssen, Dr. 
who from a study of the habits of the clouds and their 7^"^^^- 
prevailing drift, had concluded that the neighbourhood of 
Ootacamund was not the best that could be chosen. 
He had consequently taken his departure, and it seemed 
^ first as if his whereabouts was known to no one. At 
l*st, however, Professor Respighi and myself came upon 
his spoor; he was at Sholoor, on the N.R flank of the 
'^'^ge, at the solitary house of a tea-planter, to which there 
^*s no road, but which might be reached on ponies if a 
S^ide to it could be found. This guide Captain Sargeant, 
^^ the Revenue Department, obligingly provided, and in 
^^ very long time we reached the beautiful spot which Dr. 
J^lssen had chosen. It will be better that I should state 
"*5 results in his own words. In a letter^ to Professor 
*^^ la Rive, dated December 26, he thus writes : — 

**J*ai ^t^ favoris^ par un ciel d'une puret^ presque 
^^solue. * Cette circonstance, et surtout Ics dispositions 

'' Biblioth^que Universelle," January 15, 1872, p. 103. 



SOLAS PItYSiCS. 



optiques toutes nouveiles quej'avais prises, m' on t pennis i 
de faire sur la couronnc dcs constatations qui d^niontrcnt j 
son origine solaire fpour la oieilleuFe parti«). „ 

'■ Dans mon telescope,' le spectre de la couronne s'cst ■ 
montre non pas continu, mais remarquablemeRt coniplexcil 
J'y ai constate : 

■■ Les raies brillantes du gaz hydrog^e qui forme 1^ 
principal element des protub^iitnces et de la chromosphire. 

" La raie briUante vertc dcja signaiee aux ^Hpses d^ 
t S6g et I S70, et quetques autres plus faibles. 

" Des raies obscures du spectre solatre ordinaire, I 
notamment D. Ces raies sont beaucoup plus difficiles ai 

" Jles obsenations prouvent que, independamment dcs 
maticres cosmiques qui doivent exisler dans le voisinagc 
du Soleil, il existe autour de cct astre une atmosphere 
trus titendue. exccssivement rare, a base d'hydrogene. 

" Cette atmosphere, qui forme sans doute la dcrniire I 
envcloppc gazeuse du Soleil, s'alimente dc la matiere dcs 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 359 

^t "Will be seen that the importance of the brilliancy of chap. 
^"^ image, so strongly insisted upon by the eclipse Com- ^^'"- 
°^*ttee in their Instructions, had been fully recognised by 
*^*'« Janssen, whose instrument had more light even than 
^**Osc used by the British parties, who used " Browning- 
*^ith" reflectors of 9J inches aperture, and some 6 feet 

-Although my account, in this place and at this time Hydrogen. 
*^x>only be of the most general character, the coincidence 
^l^tained by Janssen, Respighi, and myself on one point 
"^^.y be briefly referred to, namely the distinct proof ob- 
^ined by each of us that above the most vivid chromo- 
spl^eric layer, and even the prominences, we have hydrogen 
^th its most familiar bright lines, and with much of the 
"structure" of its spectrum : these proofs being derived 
nc>t only from the old method of inquiry, but from the 
new one employed by Professor Respighi and myself. 

We spent the remainder of the day at Sholoor in 
"Counting the hill at the back of the house to see the 
ol>servatory, and to admire the wonderful view of the 
pl^a^ins of Mysore, which was visible between a break in the 
Mils; while the immediate neighbourhood, with its water- 
'•^lls, massive peaks, rocks here, and patches of wood 
^^re, steep ravines and tea-clad valleys, presented us with 
* Scene of perfect beauty. 

^ext morning we were away before sunrise on our 
^^y to Mr. Pogson, whom we found at the Madras Ob- 
^*Aratory. preparing to exchange time signals with the Mr. 
J^flfna party. Three photographs were taken by Mr. ''^''^ 
*Ogson at Avenashi, but the instrument used was so 
^^flTerent from those used at Bekul and Dodabct (not 
^^ mention Jaff"na) that it is difficult to institute a com- 
parison in the time at my disposal ; but it is not to 
^ doubted that they will be of the highest importance 
^Hcn the general results come to be discussed. Mr. 
* Ogson was assisted in the observations by his son and 
"^r. Chisholm, the Government architect, who was highly 



i i: Utii. 







THE ENGLISH ECLIPSE EXPEDITION, 1871. 361 




nucas. 



total m -^ commenced, fancying that the end of the world chap. 

t hand. They were under the impression that the ^^'"' 

of the Expedition with assistants and all here during Rakoo 

lipse were going to get into a balloon and off to the '»^»''- 

xid not return. 

"%vill thus be seen that the hopes of those interested Genmu 
^ various expeditions of this year have not been dis- 
^ppoixited. The composition and structure of a part of 
^c ^:^orona have been for ever set at rest, while we have 
scvcri^teen photographs, taken by instruments of the same 
^^^r and pattern, to compare with each other — eleven 
VaVeu at the ends of a base line some 400 miles long, and 
SIX at an intermediate elevated point, whereby it was hoped 
*o test the influence of the atmosphere on the observed 
P'^^nomena. Whether the slight mist will have prevented 
*'^»s or not remains to be proved, but anyhow here is 
^ Vrealth of records unequalled before, and we may 
X.t ''Op^ tQ learn much of the outer coronal regions from 
f "^^Jr comparison, not only inter se, but with Mr. Holi- 
"^i^*s admirable drawings, showing considerable changes, 
^^Hich have also come to hand. 



MXPBDmOU, 1871 



. aoK H aB who observed tbc 

rt It Ae ncariieis of die {aitf 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 363 

efore I proceed to discuss the work done by the dif- chap. 
nt parties, it will be desirable to give an idea of the ^^'^' 
ngements, and for this purpose I have prepared several 
)s, which will enable you to see what the British Associa- 
1 parties did. 

n the first instance I may remark that the weather Weaker 
ditions were somewhat problematical. Another point ^<f'*^*^^^» 
3rreat importance was that much of the ground was 
unately occupied, and it was essential when placing 

parties to bear these two considerations in mind — 

possibility of bad weather, and then the importance of 
arranging matters that if some of the observers were 
ided out, belonging to our parties, then the story might 
::ontinued by other observers. 

iere we have a map of India, which gives you a general 
I of the path of the shadow during the eclipse. The 
dow, you see, strikes India on the western coast, and it 
s down in a south-westerly direction, and cuts the 
them portion of Ceylon. 

Vhen we arrived in India we found that the Indian Local 
ervers, consisting of those well-known men Tcnnant, ^^J^^^'« 
rschel, Hennessy, Pogson, and others, had determined, 
n their knowledge of the climatic conditions of India 
hat time of the year, to occupy the central part of the 
■, and also a station at a low level ; the eminent French 
'sicist M. Janssen taking up his position at the top of 

Nielgherries. We were to station ourselves either east 
vest, or both, of these parties. Whether east or west 
lid depend upon the monsoon, and the great question 
t was being discussed on our arrival was. Was the 
ctsoon favourable } 

have not time to go into the many interesting points 
ching the answer to this question ; but I may say 
rtly that what we heard was, that if the weather was 
sly to be bad on the east side of the hill range, generi- 
ly called the Ghauts, there was a good chance for any- 
5 occupying a position west of those hills. What 



364 SOLAR PHYSICS. 



CHAP, happened was that we did occupy the positions marked by 
^^'^' blue wafers on the map, namely, Bekul on the west coast, 



Poinis Manantoddy on the western slope of the Ghauts, Poodo- 
chosen. cottah in the eastern plain, and in the island of Ceylon, 
first, Jaffna ; and secondly, Trincomalee. 

Such were our arrangements. The parties were statkmed 

along the line of totality. Very different were the a^^ang^ 

ments of the Sicilian party of the former year. In Sicily 

we were compelled to throw ourselves across the line of 

totality in the direction which I have indicated on this map 

of Sicily. 

iVork to be Now what was the work we had to do } If you will 

^"^* allow me to refer to two or three results of the fonner 

Eclipse expedition, I will endeavour to put them befoit 

you without taking up too much of your time. 

First One of the most important among the results obtained 

^'''' in the eclipse of 1870 was this: far above the hydrogti 

which we can see every day without an eclipse — far abo« 

the prominences^ the spectrum of hydrogen had witho* 

doubt been observed by two or three of the Americai 

observers, who were more fortunate than we were. Among 

them Professor Young stated, that the spectrum of hydio- 

gen was observed to a distance of 8' from the sun ; he tbtf 

adds, " far above any possible hydrogen atmospbeft 

This is point number one. 

Sffond Another of the points was this : the unknown suhstaflff 

Z*""'. which gives us a line coincident, according to Young, *!* 

a line numbered 1474 by Kirchhoff, had been observed M 

the American observers to a height of 20' above thcli^'^ 

of the dark moon. 

Now, it was a very obvious consideration that if we g''^ 
a spectrum of hydrogen 8' from the dark moon, when** 
thought we knew that the hydrogen at the sun did ^ 
really extend more than 10" beyond the dark moon,tb^ 
was something at work which had the effect of makii{ ' 
appear very much more extensive than it really was* 
and it was fair to assume that if this happened in the case 



THE ENGLISH ECLIPSE EXPEDITION, 1871. 365 



qT the hydrogen, it might also happen in the case of the chap. 
unl<nown stuff which gives us the line 1474. *^'^' 

In support of this view we had one of the few observa- 

ttons which were made in Sicily, in the shape of a drawing 

or the corona, as seen by Professor Watson, who observed 

at Carlentini. He saw the corona magnificently ; and 

being furnished with a powerful telescope, he made a 

most elaborate drawing of it, a rough copy of which I will 

throw on the screen. You will see at once that we had 

in this drawing something which seemed to militate s^inst 

the idea that the 1474 stuff at the sun did exist to a height 

of 20'. According to Professor Watson the boundary of 

the real corona was clearly defined, its height being far 

under that stated. 

Next, we had another observation of most important l^i'^ 
'scaring on our knowledge of the base of the corona. I ^'^ 
J'efcr to the announcement of the observation by Professor 
*^oung of a stratum in which all the Fraunhofer lines were 
^'cversed. It was asserted that there was undoubtedly a 
'"^'on some 2" high all round the sun, which reversed for 
''^ all the lines which are visible in the solar spectrum. 
*^*s had in fact in a region close to the photosphere the 
^^niosphere of the sun demanded by Kirchhoff at some 
uista.nce above the photosphere. 

L^.st, not least, we had the photographic evidence. There Phoio- 
^^ in Sicily a photographic station at Syracuse, and the ^^^ ^' 
Am^^cans had another in Spain. I now show on the 
scre^^ a drawing — it is not the photograph itself — but a 
dra^^ring of a photograph made by the party in Sicily ; 
what we have on this photograph, is a bright region round 
the cJark moon, which is, undoubtedly, solar, but stretching 
out X"ight away from this, here and there are large masses 
^^ *^int light, with dark spaces between them, which have 
*^^ti called rifts. Now the question is. Is this outer por- 
^^^^ solar } 

*^aving thus brought rapidly before you some of the 



.vj.djr f^Ts/cs. 






f to bear in mnx), an^c 
oo ondi to bopc for -^ 
e 'wvKk wc had to do s 
■bee aome ncv mcth(>«j 
Mtd. vith the object <; 



e Koyil AstTononiical Societ^y 
ae to tdce cfaxi^ of an ucp^ 
- rr'^ to candoct spectroscopic observe.- 
ri i^at —1^*^ Ad mc iaBoite hoaonr, Jl 
^^ Tlie HHAlHWKWtu alone, as it had bw^n 
: rKr>xT. vx:. =11 VKf cfiaio^ aot competent to de*- 1 
I have loIdS 
: America D obsci^'W^ 
X tl>e spectrum of hydrogcc* 
: ^ tiK U^ CE^se rouad the sun. to a height o * 
= a 5pec3-L:ra of hyvbogen " far above any po^iblc^ 
*i>"ir:^?n * ai tbc smL Hcoce it was in some way rcflK-^ 
tc-i Nc'* «^^b oar ordtnarj- spectroscopic methods iC 
n *j eTctrcmely ditncult, and one ro^ht say impossible, t(» 
detemjinc nhether the light which the spectroscope an)— 
Ijicd was really reSectcd or not ; and that was the whoic 
quest-ori- 

It became necessary, therefore, in order to give i^T 
approach tc hopefulness, to proceed in a somewhat ditfcf— 




THE ENGLISH ECLIPSE EXPEDITIOS, 1871. 367 



XXIV. 



it that we get a line ? Because we always employ a line chap. 
for the slit. But suppose we vary the inquiry. If, instead 
of a straight line, we have a crooked line for the slit, then 
we ought to see a crooked line through the prism. Now, 
alloMT me to go one step further : suppose that instead of 



s7f« 



I I I 



K K \ -^ 



'*Ck lag. — The q»ectnim of hydrogen as seen with a straigiht, a crooked, and a ring ilit. 

a line, whether straight or crooked, we have a slit in the 

shape of a ring, shall we see a ring through the prism } Shape of 

You will see that we shall. And then comes this question : ^*^ 

Wivhen we work in the laboratory we examine these various 

^Kts, illuminated by these various vapours, why should it 

flot happen that if we observe the corona in the same 

^y, we shall also get a ring built up by each ray of light 

^hich the corona gives to us ; since we know, from the 

American observations, that there were bright lines in the 

*P^c^rum of the corona, as observed by a line slit t In 

®"fe«" words, the corona examined by means of a long 

traio of prisms, should give us an image of itself painted 

"X ^ach ray which the corona is competent to radiate 

^ow let us pass to the screen, the screen merely re- 
P^^i ng the retina. We will first begin with the straight 
*"^ Avith which you are familiar — we now have our sht 
»»rly focussed on the screen — we then in the path of the 
"^ni interpose one of these prisms, and there we get on 
^^ screen a bright line. 



that you see we can use any kina oi narrow a 
choose, and as long as we are dealing with ligl 
monochromatic, or nearly so, we get an image i 
tiirc on the screen. 

If we consider the matter further it will be i 
we may employ a mixture of vapours, ao^fl 
result. i 

We will now, for Instance, instead of employ 
vapour, employ a mixture of various vapours 
now that each ray given by these substances 
building up a line image, is building up foi 
image — that we have now red, green, yellow 
violet rings. 

Now that was the consideration which led t 
tion of one of the new attempts to investigate 
of the corona used this time. It was, to use 
prisms, pure and simple, using the corona as 
lai^e number of prisms being necessary to s 
various rings we hope to see, by reason of t 
dispersion. On the screen the rings to a cer 
intersect each other; and it would have bee 
show you the ring-form of the images if we 
used more prisms than one. 

If this is good for a train of prisms such 
referred to, it is good for a single prism in f 
object-glass of a telescope. Such was the meth 
by Professor Respighi, the distinguished Dire 
Observa tory of the Caoitol of Rome, who m 



THE ENGLISH ECLIPSE EXPEDITION, 1871. 369 



diiromosphere would be of the same height, while if re- chap. 
fle^cion were not at work the rings would vary according ^^'^ ' 
to tilie actual height of the vapours in the sun*s atmosphere, 
and the question would be still further advanced if the 
q)e<rtnim did not contain a ring representing the sub- 
stance which underlies the hydrogen. 



ur ff^zc/ spectroscopic equipment then was as follows : — Newequtp- 
I . A train of five prisms. ''•'^• 

^. A large prism of small angle placed before the object- 
giaLss of a telescope. 
3- Integrating spectroscopes driven by clockwork, 
4, A self-registering integrating spectroscope, furnished 
with telescopes and collimators of large aperture, and 
prisms. (This instrument was lent by Lord Lindsay.) 



N'ow a word about the polariscopic instruments, referring PolaH- 
yovi to my lecture given last year for a general notion of //^J^X. 
the basis of this class of observation. 

-A. new idea was that observations to determine the 
polairization of the corona might be made with the same 
telescope and eye, both with the Biquartz and the Savart. 

By the kindness of Mr. Spottiswoode, who has placed 
hj* magnificent polarizing apparatus at our service, I hope 
to l>e able to show you on the screen the mode of examin- 
'Off the corona by means of those two instruments, so as 
to Enable you pretty well to follow what was actually done. 

l-^et me begin with the Biquartz polariscope. In the The 
"fst instance I will throw on the screen a representation of ^•9'^^*- 
t"^ corona itself, and we will then insert a Biquartz, and 
*^^ its effect when I flood the screen with polarized light. 
^^v^ now see an indication of what would be observed 
^^'t^posing the polarization was due to polarized light dif- 
^^'^^d in the region between us and the dark moon and 
^^ipsed sun ; in which case the polariscopic effect would be 
^»^^rved generally over the dark moon, the corona and the 
'^ion of the sky outside the corona. But this is not all, 
'^^^ only does this arrangement enable us to determine the 

B B 



370 



SOLAR PHYSICS. 



CHAP. 
XXIV. 



The 
Savart. 



kadial po- 
larization. 



existence of such a general polarization, but the vertical 
line in the Biquartz called the line of junction, when the 
colours on both sides of it are the same, indicates the 
plane of polarization ; so that we have two colours strongly 
contrasted in either half of the field when we arc away 
from the plane of polarization, and a uniform colouring 
of the whole field when in or at right angles to that plane. 
By turning this prism through 90**, you see I entirely 
change the colours. 

But we are not limited to the Biquartz in this inquiry. 
We can apply the Savart polariscope. Having still our 
image of the corona on the screen, I now replace the 
Biquartz by a Savart. 

We now no longer see a line of junction with the similar 
or different colours on either side of it, but lines of colour 
running across the image. I turn the prism. We first sec 
the lines with a white centre, then with a dark one : while 
at times they are altogether absent. And as a departure 
from the plane, when we use the Biquartz, gives us the 
strongest contrasts of colour, so you observe that with the 
Savart under these circumstances all indications of polari- 
zation vanish. 

Now, if we assume polarization to be general, and the 
plane of polarization vertical, we should get those coloured 
bands, as you see them there, crossing the corona and dark 
moon, the lines being vertical and dark-centred. If the 
plane of polarization were horizontal, we should find the 
lines horizontal and the central one white. 

But so far as we have gone, we have been dealing wi4 
polarization which is general, and we have not attcmpWl 
to localize polarization at the corona itself. But I have 
here an apparatus, by means of which, quietly, in thii 
theatre one can see as admirable an example as we sbodd 
desire of polarization assumed to be particular to the stf 
and not general — I mean radial polarization. Wc hut 
simply a circular piece of mahogany, or something dse 
which polarizes light equally well, with a hole in the ffliddk 



THE ENGLISH ECLIPSE EXPEDITION, 1871. 371 

with sloping sides, cut as you see this cut, and then we chap. 
place behind it a candle, so that the light of the candle, ^^^Z:__ 
after falling on oiled tissue paper stretched across the 
aperture, can be reflected to the eye by the sides, the How 
direct light of the candle being stopped by a central me- ^^^^^'f^- 
tallic diaphragm. We have now a source of polarized 
light of a different kind from the last. The next thing we 
have to do is to introduce into a small telescope exactly 
the same kind of apparatus we have there, though of 
course on a much smaller scale, and examine the ring of 
light seen when we put the candle behind the aperture. 
On examining the ring of light which is now visible by 
means of this telescope, which contains a Biquartz and 
analyzer, I see the most exquisite gradations of colour on 
either side the line of junction which cuts the field of view 
and the bright ring in the centre into two. 

Now, instead of the candle, we will employ the electric 
lamp ; and instead of the eye, the screen ; but I must 
inform you that the great heat of the electric lamp 
prevents the appearance being perfectly successful on the 
screen, as the reflecting varnish is melted. 

In this experiment we cannot work with an image of 
the corona. We must make our corona out of the image 
of the ring we hope to get on the screen ; and then, by 
employing the Biquartz in the same way as before ; instead 
of getting similar colours on either side of the line 
of junction, as we did when we were working in the plane 
of polarization, and getting the greatest contrasts, as we 
did when we worked 45"* away, you observe we get 
different colours in each part of the ring. 

On the screen we now have a highly-magnified image Expert- 
of the hollow cone of iron which I am compellincf to reflect .'"^'''^^ 
the light from the lamp ; and by mserting this Biquartz twn, 
I throw various colours over different portions of that 
ring, which I beg you to consider for one moment as the 
nolar corona, and the colours change as I rotate this prism. 
You will at once be able to explain the different actions 

B H 2 



372 SOLAR PHYSICS. 



cHAF. of the Biquartz in this instance. The reflexion, and 
^^'^* therefore the plane of polarization, is no longer general, 
but varies from point to point of the reflecting surface 
It is in fact radial, and hence the delicate radiate arrange- 
ment of colour. 

Such then were some of the new methods and new 
instruments we used for the first time in our researchei 
And I hope you will allow me to use this term, although 
our work was conducted a long way from the Royal 
Institution, the natural home of research in England. 

Thecertain I must now statc vcry briefly some of the results of oa 
results, work ; and first, the certain results. 

Structure We Were able to make out the structure of the corota 
of corona, y^ ^ know all about the corona so far as the structure 
its lower brighter strata, that portion, viz., which I referr' 
to in my lecture last year^ as being visible both before a« 
after totality, is concerned. You may define it as co 
sisting of cool prominences ; that is to say, if you exanu 
a prominence any day, without waiting for an eclipse, a 
then go to an eclipse and examine the lower portion 
the corona, you will find the same phenomena, minus 
brightness. You find the delicate thread-like filam 
which you are now all so familiar with in prominenc 
filaments which were first thrown on a screen in 
theatre; the cloudy light masses ; the mottling ; the neb 
structure, are all absolutely produced in the corona, . 
as I could see it with a telescope with an apertuP 
inches ; and I may add that the portion some 5' rou; 
sun, reminded me forcibly, in parts, of the nebula of 
and of that surrounding 17 Argus, as depicted by Sf 
Herschel, in his Cape observations. 

We have shown that the idea that we did 
hydrogen above 10' above the sun is erronec 

1 See p. 281. 



THE ENGLISH ECLIPSE EXPEDITION, 187 1. 373 



^ we obtained evidence that hydrogen exists to a height chap. 

1^ of 8' or 10' at least above the sun; and I need not ^^^^' 

; tell you the extreme importance of this determination. 

One of the proofs we have of that lies in this diagram, Hydrogen 
siiowing the observations made by Professor Respighi, ^^^^ 
armed with an instrument, the principle of which I hope formerly 
yoxx are now familiar with. ' 

J'ust after the sun disappeared Professor Respighi 
^*nf>loyed this prism to determine the materials of which 
tti.^ prominences which were then being eclipsed were 
CO rx^ posed ; and he got the prominences shaped out in red, 
''^ yellow, in blue, and in violet light ; a background of im- 
P^ir*^ spectrum filling the field; and then, as the moon 
s'^^^^pDt over the prominences, these images became in- 
ile ; he saw the impure spectrum and the yellow and 
it rings gradually die out, and then three bright and 
rings, painted in red, green, and blue, gradually 
in the field of view of his instrument ; and as long 
le more brilliant prominences were invisible on both 
of the sun, he saw these magnificent rings, which 
**^*"^"W him in a state of ecstasy. And well they might. 

I^liese rings were formed by C and F, which shows Notglan. 

**^ tliat hydrogen extends at least 7' high, for had we 

Hen dealing with hydrogen we should have got a yellow 

^ as well, because the substance which underlies the 

. ^^^ogen is more brilliant than the hydrogen itself, and 

^^ ^^^xldition to the red ring and the blue ring, which indi- 

the spectrum of hydrogen, he saw a bright green ring, 

^ Ji more brilliant than the others, built up by the un- 

*^^^X3vn substance which gives us the Kirchhoff line, 1474. 

, "^^ow at the time that Professor Respighi was observing 

^^^ beautiful rings by means of a single prism and a 

^^cope of some four inches aperture, some 300 miles 

^ ^"3^ from him — he was at Poodocottah and I was at 

• ^xil — I had arranged the train of prisms which you see 

*^ ^ so that the light of the sun should enter the first 

^n, and after leaving the last one should enter my eye. 



SOLAR PHYSICS. 



MiAp. And \vhat I saw is shown, side by side with Respighi's 

^"■'ll observations, in this diagram, in which I have separated 

CoHifiiri- ji,g i-jiigs somewhat, so that there should be less confusion 
KtitiihCs than in the actual observation. Here is Professor Respigbi's 
r"ii "hs'i- ^'^^^ observation. He gets indications of C, D', F, and 
i;i/i,'iis. the hydrogen line near G, He was observing the very 
lowest, brightest region of all, and therefore 1474 was 
obliterated by the brightness of the continuous spectrum ; 
bLit as the eclipse went on D' was entirely obliterated, and 
fterwards he got C and F building up rings together with 
1474, which was not represented in the lower r^ions of 
the prominence — not because it was not there, but because, 
as I have already insisted, of the extreme brilliancy 0' 
the background. Now my observation was made inter- 
mediately, as it were, between the two observations of 
i'rofessor Respighi's. Let me show the observations 
together. 

Rcspiijhi .CD* F G Prominences at beginning 

of eclipse. 




.=' ■^■:.=^i75 



I 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 375 

he^ht above the more intensely heated lower levels of the lhap. 
aomosphere, including the prominences in which the ._''i'"'i_ 
vapours are thrown a greater height. With a 
Kcope of small dispersion attached to the largest 
ror of smallest focus which I could obtain in Eng- 
lOd, the gaseous nature of the spectrum, as indicated by Gasum 
y structure, that is, bands of light and darker intervals '/>«'"'"■ 
Ldbtinguished from a continuous spectrum properly so 
id, was also rendered evident. 

z are results of the highest importance, which alone 
* worth all the anxiety and labour connected with the 
Ipcdition. 
\ But there is more behind. The photographic operations piuitv 
t of the expense of which was borne by Lord Lindsay) ^''^J' 
: most satisfactory, and the solar corona was photo- 
raphed to a greater height than it was observed by the 
sctroscope, and with details which were not observed in 
e spectroscope, 

Mr. Davis was fortunate enough to take an admirable 
lies of five phot<^raphs at Bekul, and Captain Hogg 
I obtained some at Jaffna ; but I am sorry to say the 
r lack somewhat in detail. 

have prepared two lamps, because I am anxious to 

ibit the phot<^p^phs two at a time, that you may 

■re one with the other. [This was done.] You see 

» far as the camera goes — and mark this well — the 

, was almost changeless during the whole period 

■lity ; this is true, not only for one place, but for all 

%s at which it was photographed. 

exhibit two other photographs — one taken at 

\ and the other at Ootacamund. Actinically the 

fl was the same and practically changeless at all the 

You see that, though not so obvious as in the 

Vlcasc, there is the same similarity. 

E I leave the actinic corona, I am anxious to show 
di image of it, taken during the American eclipse of 
jia a camera exposed to the sun during the whole of 



:,0 LA R PHYSICS. 



the totality ; to a certain extent in our recent photographs 
. ive have reproduced what was phot>:^raphed in 1869. 
Tiie iolar nature of most, if not all, of the coram. 
recorded on the plates is establisbed by the fact thai the 
plates, taten in different [^aces, and both at the beginning' 
and end of totality, closely resemble each other, and miidt 
of the exterior detailed structure is a continuation of that 
obsen'ed in the inner portion independently determined bj- 
the spectroscope to belong to the sun. 

While both in the prism and the 6 inch equatorial the 
corona seemed to form pretty regular rings round the dirt 
moon, of different heights according to the amoiiiit of 
li^ht utilized by the instrument ; on the photographic plaice 
the corona, which, as I have before stated, exccedi the 
limits actually seen in the instrximent I have named, has » 
verj' irregular, somewhat stellate outline; most matkcJ 
breaks or rifts {i^ioreJ by the sfieclroscope), occurring near* 
the sun's poles, a fact perhaps connected with the other 
fact that thi; most active and most brilliant promincncrs 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 



377 



m 
act 



eye 
lin 



e photographs, though in places the shape of the 
ic corona and some of its details are shown, 
ow the corona, as it appeared to me with the naked 
was nothing but an assemblage of bright and dark 
it lacked all the structure of the photographs and 



CHAP. 
XXIV. 

Naked eye 
corona. 



app <z,a red larger ; and I have asked myself whether these 
line^ do not in some way depend on the size of the 
tel^^<:ope, or the absence of a telescope. It seems as if 
obs^^ aerations of the corona with the naked eye, or with a 
tele^s-^ope of small power, may give us such lines ; but that 
whc Ai we use a telescope of large power, it will give, close 
to ^^l^e moon, the structure to which I have referred, and 
abolish the exterior structure altogether, leaving a ring 
rouxxd the dark body of the mooh, such as Professor 
Re^pighi and myself saw in our prisms and in the 6-inch 
telescope, in which the light was reduced by high magnifica- 
tion, so as to bring the corona to a definite ring some 5' 
bigl:^^ wh|le Professor Respighi, using a 4-inch telescope 
and less magnifying power, brought the corona to a ring 
sonr^ething like f high. 

A.iid here we have an important connection between 
speotroscopic and telescopic work. If we employ a tele- 
scope in which the light is small or is reduced by high 
magnification, we bring the corona to a definite ring, and 
perhaps here we have the origin of the * ring-formed * 
coronas. 

Many instances of changing rays, like those seen by 
Pianta.tnour in i860, were recorded by observers in whom 
1 nav^ every confidence. One observer noted that the 
'^ys revolved and disappeared over the rifts. 

^^ have next to deal with the polariscopic observa- 
tions. 

^'"* Lewis, in sweeping round the corona at a distance 

^ 7 Or f from the sun's limb, using a pair of compen- 

^ ^'^S quartz wedges as an analyzer, which remained 

I^^'^Uel to itself while the telescope swept round, observed 

^ands gradually change in intensity, then disappear, 



Possible 
origin of 

ring- 
formed 
coronas. 



Polarisco- 

pic observa* 

turns. 



rdaiacSeriftenFanls appealing, 

[ bans of atno- 
sihsn^ taz a^:ae df adU. palmiaboa, with a Sanit 
I of '■' ■ ■* Ae result obtained by 
vUe DO tomiag in the 
I of the image of !k 
; faint traces of ndial 
(or a short distance from 
I:^^ -SKcc : T — gt ^~frt*^ TDpatm. vho observed nitli 
=•£ p>:I^ri~c:: zc aAo' tobfity. announces stroi^ ndiil 
• ^s~zai^~ ^T^a£ag to a verf considerable dtstaoa 

Le27=^ :^t I I in ■!! fMtadc of the corona as a quc^ 
at some future time — and it cu 
: to the base of the coroaa, ui 
wtidi I have alreadv refcrrfii. dose 



V. zi: ~7Lf '.ne ge o ci al oondttsion at which we anixti 




THE ENGLISH ECLIPSE EXPEDITION, 1871. 379 

sec very many more bright lines than we do when this chap. 
ot the case, the lines being of unequal height. ^^'^* 

[r. Pringle, also at Bekul, showed that, at the end of 
Jity many lines flashed into one of these instruments, 
led under these difficult conditions, 
laptain Fyers, the Surveyor-General of Ceylon, observ- 
with a spectroscope of the second kind, saw something 
I a reversal of all the lines at the beginning, but nothing 
the kind at the end. 

dr. Fergusson, observing with a similar instrument, saw 
ersal neither at the beginning nor the end. 
Ax. Mosely, whose observations are of great weight, says 
t at the beginning of the eclipse he did not see this 
crsal of lines. Whether it was visible at the end he 
lid not tell, because at the close the slit had travelled 
the edge of the moon. 

Professor Respighi, using no slit whatever, and being 
der the best conditions for seeing the reversal of the 
es, certainly did not see it at the beginning, but he 
isiders he saw it at the end, though about this he is 
ubtful. 

From the foregoing general statement of the observa- Conclusion, 
ns made on the eclipse of last year, it will be seen that 
>wledge has been very greatly advanced, and that most 
X>rtant data have been obtained to aid in the discussion 
former observations. Further, many of the questions 
€d by the recent observations make it imperatively 
cssary that future eclipses should be carefully observed, 
periodic changes in the corona may then possibly be 
^d to occur. In these observations the instruments 
Ve described should be considered normal^ and they 
^Id be added to as much as possible. 

had intended, if time had permitted me, to point out 
^ much better we are prepared for the observation of 

eclipse now than we were when we went to India, and 
*it a system of photograph record should be introduced 



SOIAX PHYSICS. 

into tbe spectroscopic and polariscopic work; but time 
will not allow me to do more than saggcst this intere^ag 
topic I arr. anxious, boire\-er, that you should allow mc 
CDC minute -. rt to say how very grateful we fee! forlie 
asastance r^r.u.-rcd by all we met, to which assist^cc 9> 
TDudi of our s-j ccess must be ascribed. I wish thus publicly 
to express the extreme gratitude of ever)- one of oor 
expedition to the authorities in India and Ceylon for tbe 
a<~Utance we received from them ; and our sorronr thai 
Admiral Cockbuni, a warm and well-known friend to 
Science, who placed bis flagship at the disposal of tht 
expedition, and the Viceroy, whose influence in our favoui 
was felt in everj' region of India whither our parties wait, 
and to whom M-e gave up our ship, are now, alas ! beyond 
the expression of our tlianlcs. We are also anxious 
expre» our oblations to the directors and ofEccn of 
the Peninsular and Oriental Company for the magnificait 
ich they aided us. If they had not assisted 
did. Science would have gained vtry mucli 




THREE YEARS' WORK WITH THE NEW 

MET HOD > 



XXV. 



r the two lectures I delivered in this Theatre three years chap. 
^ I endeavoured to bring before you some of the first 
Kiilts which had been obtained by employing a new 
Ktfaod in solar research — that method, namely, which 
ad>les us to study now one little bit and now another of 
w sun. And in order to place these results as clearly as 
asible before you, I sketched the work which had been 
■le since the invention of the telescope, remarking that 
<:e that time not only has the power of our telescopes 
leased, but we have had first one instrument of research 
d then another added, so to speak, to our scientific 
•«k-in-trade. As I am now to place before you some of 
^ results which have crowned the efforts which have 
^n made to increase our knowledge of the sun during 
- last three years, I shall endeavour to connect the new 
•k with the old this time, pretty much as I endeavoured 
do upon the last occasion. Nor is this all. Such great 
^^nces have been made that it will be necessary for me to 
5t up the subject, in order that the different lines of 
^arch may be the better understood, and that my own 

Revised from the short-hand notes of two lectures delivered before 
Literary and Philosophical Society of Newcastle-upon-Tyne, in 
:ober 1872. 



:r.e \-anous solar pbenomena witti 
■■.i has faniiliarized us, we may g 



1- \ ir:5:io:ii in tfae positions of fines in the 
:r-jm ichanges of wax-e-Iength), cornected wi 
JTi the lines from dailc to br^ht, 

:;. \"ariatii:i:i5 in Uic thickness of spectral 1 
tive radiation and absorption), and in the amoi 
::n'JOiis absorption. 

A moment "s thcnight, however, will convinci 
this grouping is too general ; and that it will fa 
deal with the soiar phenomena themselves, ai 
under each head the advances made. This I si 
i: w-ill be convenient to say at the outset one i 
ally on the qi:estion of the changes of wave-I) 
comes in in ever}- psut of the inquif}'. 



Changes of Wave-length, 



rfrl 



In nij- former lectures I stated the fw/w«<tfr" 
method of determining the velocities of the vap 
solar atmosphere, and felt so certain about it th 
entered my mind that it could be questioned : ii 
questioned, however, and it is my duty to recoi 



? YEARS' WORK WITH THE NEW METHOD. 383 



*en the phenomena, but does not say whether he chap. 



XXV. 



es with the explanation. Father Secchi also for 
ime discredited it ; while admitting that he had 
phenomenon, he considered that too much impor- 
d been ascribed to it, as he said he observed it 
place all round the sun's limb, and that a little 

I had convinced him that it wa^ simply due to 

:t of t/u suits rotation. He stated that he had Stcchi. 
. displacements of ^ of the distance from D* to 

that such a movement was equal to over 300 
es per second. He then proceeded to calculate 
of the solar equatorial rotation, and made it 429 
es per second — a mistake of a most remarkable 

it is over 200 times the true rate of that motion, 
•or was immediately indicated by Fizeau, where- 
cxhi hastened to correct it, at the same time stating 

true rate of 1.92 kilometres did not in any way 
te his explanation, but, on the contrary, agreed 
rith the smallness of displacement observed by 
lisplacement which be it remembered he at first 
3uld not be produced by a rate less than 300 kilo- 
per second. But I am glad to say that Father 
las now roundly stated his conviction of the accu- 
my announcement. M. Rayet, also, in a memoir Rayei, 
ie admfrable from several points of view, has 

to my observations on the changes in question 

illusions de M. Lockyer;" but Professor Young Young. 
e first has observed and . recorded phenomena ex- 
milar to those I described to you, and has from 
: accepted my explanation of them. Still later, 

astronomer, Dr. H. C. Vogel, of Bothkamp, has Vogei. 
ed my views. So that now I think I am not 
too much in stating that this matter is quite 
[led. 

II refer to the changes of the lines from dark to 
further on. 



SOLAK PUySlCSi 



Spots GEHERAixr. 

/. dmnectiam v-itk Platutary Configurations. 

We will begin our more detailed consideration nf tJ* 
recent work wth a statement of the progress made in (X^' 
knowledge of sun-spots. These spots are already fatiiili.^' 
to you ; and you may also recollect that I referred W tl^* 
ma^ricent researches of Carrington and of ihe Kew obse*" 
vei-^. which have uofolded to us nian>- facts connected *itl' 
the origin and nature of them. I also told you Ihat » 
discus^sion o! the appearances presented by the spots hiJ 
given rise to two distinct theories of their origin, and th*t 
a le^t of the most coodusive nature had been applicil by 
the new method of research. 

It was next pointed out that the Spot phenomena ftxc 
i>bvi-.'tis!y I.xated in a particular niveau of the sun's at- 
mosphere. arj that probably there might be conditions 




HREE YEARS' WORK WITH THE NEW METHOD. 385 



similarly, if at one time we get iron perpetually chap. 
jght before us in the spots, and at another time we get ^^^' 



\ um, then, in those cases, we should get a difference due to Secular 
*, as in the other case we got a difference due to place, ^arit^ons 
ther, I pointed out to you that from the hydrogen to observed, 
iron there was a running down in the rate of motion 
lie vapours, and that, associated with these indications 
general and of varying selective absorption, we had very 
n a perfectly distinct phenomenon — that is, the sudden 
litening of many of the lines in the spectrum : the 
s of hydrogen, sodium, and magnesium, for instance. 
n what I said about the sun-spots I was careful to refer 
he work of Carrington and Balfour Stewart, De La Rue 
Loewy, work which has largely increased our know- 
je of the distribution and motion of the spots, so that 
knowledge of the sun-spot curve — one of about eleven Thesun- 
rs period, during which we have no spots on the sun, a ^^^ ^^^'' 
spots, and then a maximum of spots ; coming back after 
'en years to the period of no spots, or of minimum sun- 
ts again — is rapidly getting more exact. Our know- 
re of the true sliape of this curve has, for instance, been 
^ased by some researches of Wolf and Fritz,^ from 
'h it seems that instead of running over a period of 
sn years and i-ioth, the period is only of eleven years 
7-lOOths. That, you will say, is a very small matter. 
•ther small matter, but one which in the long run will 
>ably prove to be of considerable importance, is, that Rdations 
descending: part of the curve is always associated, and ofvatious 

o^ . , , . ,. . r 1 partsofthe 

^s with the previous ascendmg portion of the curve, curveto 

^cover, the curve requires three years and a half to ^^^ ^'^^• 

*nd and seven years and a half to descend.^ So that 

^n we have a period of maximum sun-spots, such as 

had last year, we may expect the spots and associated 

cnomena to be much longer in running down than they 

Te in attaining their maximum. 

1 Proc. R, S. vol. xix. p. 392. 
* Dc La Rue, Stewart, and Loewy, J'roc. K. S. vol. xx. p. 82. 

C C 



.sacjw/VKfxx 



t oonfimutioa of the 
ui=i. zxa: ?,r-^^. li ae at aaae «a 7 ooQoected wilh pliue- 
=arv ^:cicrL--^^:^c& amA it knls nsy nwdt as if— 1 »y 
iz jiKxs '£;/.' - -iir iBotai^jr ■anj' of you will not 
^fDSiiJir "ie rT-..n a« as jet eoadBSvc, — most oi the 
jatmznn^sL :>^<e^ai ^pas soMK ■Jocnce csurted by Veiui^ 
i^:xi JL=r=ir7 Iz A p>pcr wHA has been comrouniutc^ 
rr -ziii j-jyri 5>:ce«j- dbnuc •^ present year, the Kf**' 
.c-e-T-r^ szi::v zjsx Ac m 1 n,i siie c( a spot icDuld 
zpxs^ 1=: frs^^a ::3 ^hsm^b (M that side of the sun «hicli 
f^ t^T'iei £«;^7 i - ^ ia Vohb or fnaa Mcrcuiy, and lo have 
x^ -mrrrmiTTT ji t^ i.i^HwiWiiiilcrf Venus or of Mcn:ur>'< 
Zi .x^c v«-^i^ir=;diag«iA tbe put of the sun-spot action 
v::ic=r ^ c.je r^ Vean^S'jm lived on the planet Venus 
:- X. v.-iiJi arrcr soc a 1^ 'pm ; and dealing with that 
-•irnrc wiu::i rsis-s t3 the planet Memin-. if yoo \ivei on 
m; '■"■''*'" M tr;-ry voa would never see a sun-spot— so 
:; : ^ "<: 1 w^ ai to be fbnnd 00 the side of the $aa 
\zi ^ r^TTi-- is-aTftoBVeiiBs orfromMeTCury. natuially 




THREE YEARS' WORK WITH THE NEW METHOD, i%7 

the explanations of the new phenomena observed, which chap. 
have been given, not only in this part of the inquiry but _^*^* 
in others, as you will see as we go on, have not been uni- 
versally accepted. For instance, instead of referring the 
phenomenon presented by sun-spots to the continuous 
absorption and to the greater density of particular vapours 
in the sun. Father Secchi has announced that the change seeches 
from the ordinary spectrum is due to the presence of *"^*'* 
aqueous vapour in the sun-spots (see Note A). This con- 
clusion I have not been able to endorse. And it has also 
been stated by another Italian observer, Professor Respighi, RespighCs 
that the various selective absorption phenomena presented '^'"^' 
by sun-spots may depend upon our instruments, and is 
not real. 

Passing from questions which have been discussed, we 
have a great deal of new work to place on record. 

You already know that the thickness of a spectral line 
is an indication of the pressure of the gas or vapour which 
is radiating or absorbing the light. For instance, if we Thinness 
consider the lines D in the ordinary spectrum of any part thUkness 
of the sun, in a .spot we find these lines considerably thick- of lines = 
^ed; and if we get the vapour of sodium not absorbings 'pi'e^su!T 
*s it does in the case of a spot, but radiating as it does 
'H a prominence, we neither have the line of medium thick- 
^css, as you see it in the photosphere, nor of considerable 
^■ckness, as you see it in a spot, but of an almost incon- 
^^vable thinness. Now then, the spectroscope has afforded 
*■* evidence that the sun-spots are certainly not what they The 
Were three years ago. The spectral lines over the umbra ^^^^'i*^^ 
^^ I>enumbra are very much thinner ; in other words, the it was 
indications of absorption, both continuous and selective, '^^'^^y^^' 
^^'^ Very much less. Of course, in the telescope the record 
would run that the umbra and penumbra are much darker 
*^ ^ome times, probably near the sun-spot minimum, than 
^ others; and in a conversation which I had with 
^^- Howlett, one of our most indefatigable sun-spot obser- 
^^tB, on the question, some time ago, he stated it as his 

C c 2 




nur^ffaoa zhsc TTiis s «gt Iff a Ioub*'' 

^^1^ VIZ xxvK addlEd K anr baack id reseaicfa in our 
?r.^es ::t trrc siaspdt qfde, ^ole ia dq>e n dait of tlie 
■zii^rt iiuuber : f ;p<^ ■!>■* ^^ "tt^ >BiI <> >> probably tiic 
:x:sz a.aas oiac vc ftw^Ar adibe "^^ dements to Ihcec 
-i-iiiiih Tc aiicaiiy k^^^Bb axBC ia the iun's atmosphere. 
inC :(* <ieSeraniai^[^^^K^^B^ ftotAaas of the higher X: 
jir^r^sc tux Qolr anVwUnaoBcr but to tbc chemist: - 
3uc rt *tuve o::)ae voT' ■■■:& ■(■« tban this. la this cbs^ 
cf ^C9crf2C>.-c5 we b¥e 4iee asm pcMots to be attendeaJ 
U dr^ SK lines wkick arc thi cke ned, as tirst obfiened b>'' 
ca^la 14*56; stLXttlioacwhicbaiTbnghteiiedbyproinineaccrs 
ijaC25 r-Ti-er ^jr-ts' (Apdi i&6q', and lastly the indications 
ii* Tn:t::ic bei:rs r e fe ngd t(X. 

X:^ '*ir!i regard to the fiist point : to sodium, magoesiuin. 
ac- b<;:";re ir^rntiooed. Father Seocfai soon added caldun* 




THREE YEARS' WORK WITH THE NEW METHOD. 



3«9 



which I am acquainted. Allow me to quote from these chap, 
observations.' ^^^- 

On September 22nd, 1870, Professor Young saw the 
sodium lines D, and D, reversed in the spectrum of the 
umbra of a large spot near the eastern h'mb of the sun. 
At the same time the c and F lines were also reversed. 

The figure gives the appearance of the sodium lines. In Ram-saie) 

the umbra of the spot the D, line was not visible, but in the ,:™*"'" 

* ** ' ttna ever 

penumbra was plainly seen, as a dark shade, represented in a tpet. 
the hgure. 



n» 



"3 



" In the spot-spectrum the magnesium lines b^, 3„ and b^ 
icere not reversed, but while the shade which accompanies 
fie lines was perceptibly widened, the central black line 
'tself was thinned and lightened," 

Jn another communication he gives a large list of lines 
•Ji'ch he observed in several of the spots which then ap- 
peared upon the face of the sun. He states that once when 
*"* magnesium line d showed that vapour to be in a 
"ite of perfect rest, the included nickel line indicated , 
f'ormous motion by the changes of wave-length which ' 
*' Underwent, from a cause which has already been com- 
pletely explained to you.* He then also states that, in the 
***« of some of the spots he has succeeded with absolute 
****tinctness in locating in the prominence near the spot the 
**gion3 to which the various vapours were confined. Thus 

* Journal of tkt Franklin Institute, 1870, p. 232,7. 
—^ ' Communication to the Journal of the Franklin Institute, dated 
^tobetjrd, 1870. 



^le line m tite yellow pact of l&c ^pcctnun. which some 
mti^esriid :a ziie:ie m^cLciB at31 ciMwidn' to be a line due 
:o ilvdr^lgtl3, wo:} on one ^K^ a vijt^ cwifiaeil ta the ccnEnl 
part Q(' 3. proauneac^ over a satk-apot, while be saw the 
jyiTi'^ea .^ei -ist-iiding brtfeynod this regioa. Thitis 
i pieo: of uiipuna::: evidcEcc tn a paint which hAs \xa\ 
'juchetl iijjiia la Laooratocy expenmeats, and in thelisC 
laii.Mi cxlip.ie. :o whiidi I dull again refer. I nuy her^ 
Add tkat we oiii^t cji*- asBocBtc. I Uunlc, these bright pro — 
Tvii.idaij^i. built up it' v3fOitTS ^viog OS varkius spectn.* 
..ces, *iiii the cjiours which now for many yean past (wv^ 
beeti t;iciCop(i:aIlv obsenped on the umbra: of spots iw"* 
i:i.v. Anicrica. ina Eoglaad ; and gneai credit is due i*^ 
Filliiir Secciii i-jf giving gieal anestitio to this point in hi -* 
r e£c a :ly publish ed work. 

I w\i[ now pas.^ -m froa the aiiisiderat>oa of the ^t^^i 
,-.r;<i in dumg sO, I must a^ yoa Iq bear with mc if f.^/ 
-: irj- ieemi too dctiiied, for as any branch of science ai* ■ 
, ^r.ccs, the frc^; bo! J strokes by whicli on* niay very eiisil_^S 




ZE YEARS' WORK WITH THE NEW METHOD, 391 

I the sun's atmosphere, had been discovered ; and I chap. 
»wed you that in addition to that, the new method ^^^' 
\ indication of a general welling-up of the lower 
— magnesium and iron, for instance — from time 
And I also lastly pointed out that the promi- 
lad been associated with a stratum which was con- 
round the sun. Now, although these conclusions 
»t all been readily accepted, I think we now may 
t the facts which I pointed out to you in 1870 
I of them been established. 

Young has succeeded in obtaining photographs 
)tuberance, of which he has been good enough to 
me a specimen. It was obtained by attaching 
camera to the eye-piece of the telescope and open- 
slit somewhat widely, using the hydrogen line 

He adds: — "As a picture, the little thing Youn;: 
5 to nothing, because the unsteadiness of the air and ^aphs 
ladjustment of the polar axis of the equatorial apromi- 
the image to shift its place slightly during the long '"''"^^' 
e of three-and-a-half minutes which was required, 
stroying all the details. Still, the double-headed 
the prominence is evident, and the possibility 
ig such photographs is established."* 
le present moment it is impossible to predict what 
nate form of such researches will be, but it is clear 
ore long we shall have a photographic daily record 
>lar phenomena. 

bllowing is Professor Young's account of his work, as com- 
i to the Journal of the Franklin Institute^ Oct. 3, 1870. 

irotuberances are so well seen through the F and 2796 {near 
that it is even possible to photograph them, though perhaps 
ictorily with so small a telescope as the one at my command. 
>enments I have recently made show that the time of expo- 
i ordinary portrait collodion, must be nearly four minutes, in 
produce images of a size which would correspond to a picture 
ir disc about two inches in diameter. This length of exposure 
a more perfect clockwork than my instrument possesses, 
re accurate adjustment of the polar axis than it had during 
ments, as well as a steadier condition of the atmosphere. 



SOLAR PJIVSJCS. 



"Thus far, therefore, I have not been able lo produce onyihing»hich 
could propi^rly be called a good picture Negatives have been miiic 
- which show clearly the presence and general form of protuberanctti 
but the delinicion of details is unsatisfactory. This amount of success 
was reached upon September iHth, when impressions were obtained nl 
two protuberances on the S.E. limb of the sun, and, slight as ihis 
success was in itself, I consider it of importance in showing the pccfm 
feasibility of going much further with more sensitive chemicals, moti: 
delicate adjustments, and greater telescopic power. [ was aidtil in 
the experiments by Mr. H. O. Uly, our local phoH^rai^er, to vhoir 




1 acknowledgments fur the i 
ingenuity and skill, with which he assisted me. 

" We woikcd through the hydrogen y line (2796 of KirchhoB'* «*'' > 
which, though very faint to the eye, was found to be decH^^ 
superior to F in actinic power. The photographic appnraiusetnplf"^ 
consisted merely of a wooden tube, about 6 inches long, dlt*^ 
at one end to the eye-piece of the spectroscope, and at the other «T 
ing a i'^ht frame. In this frame was placed a small plaie-b^if''' 
containing for a sensitivc-pl.itc an ordinary microscope slide. 3 iocs'* 




//. Forms and Classes of Prominences. 
There is one thing in which we have made progress, 
this is. that all the workers are now, I think, quite 





jpntent to divide the prominences mainly into two classes, 
nely, eruptive and nebulous. This conclusion, which I 
i; laken from W^e J ournul of tin Franklin !n- 
). p- 187, 



J 



THREE YEARS' WORK WITH THE NEW METHOD, 395 



brought before you in the last lecture, has been accepted chat. 



XXV. 



by Secchiy Zollner, Sporer, and by Young,^ and also by *._ 

Professor Respighi, who, however, from his enormous study 

of the prominences — he has carefully mapped and drawn 

not less than 7,000 — is inclined to sub-divide the eruptive Ktspi^hi's 

class into a great many sub-classes. "* ' ''^^''' 



///. Dimensiopis of the Protuberances. 

I have just referred to the enormous number of promi- 
nences catalogued by Professor Respighi : I now propose 
to refer to his observations on their dimensions. 

After remarking that — 

*'Oar measurements do not generally give the true height of the Muuure- 
pnilllbennces, because often their lower part is more or less concealed nwnts do 

by tte solar disc, and therefore it is only by those protuberances, the *^^ always 
•^ "of which are to be seen on the circumference or near to it, that give the 

height can be obtained." ''"' '*"^^'' • 



He adds : — 

*ln alone and continuous series of observations, since necessarily 
msny protuberances are presented in the condition alluded to, so 
finum the greatest heights obser\'cd the extreme limit may be considered 
as determined satisfactorily — that is, the height to which these masses 
of hydrogen may rise on the solar surface." 

His evidence then runs as follows: — 

"In the whole course of my observations I have never found a pro- Extreme 
tuberance sensibly higher than 6', and very few of them approach that height. 
limit.* 

" Tlie protuberances or jets which attain this extreme, and in general 
all Jticat heights, are those produced by temporary or evanescent 
erapCionty which are developed at intervals in the locality of the spots, 
wilile in other protuberances, whether isolated or combined in groups, 
vCfy leldom is the height above three minutes- about ten terrestrial 
diamelers. A moderate number of them attain the height of 2', very 
mairr more r, or more than three terrestrial diameters. 

" Without making a detailed statement of the protul>eranccs ranged 
accofding to their apparent heights, in order to furnish some idea of 



1 "About forty different prominences have been more or less 
carefully observed (from September 10 to October 3, itS/o), sixteen 
have been sketched. Most of them fill natunillv enough into the 
categories established by Zrjllner and Lockycr." Jourttjl of the 
Franklin hixtitule^ 1870. 



soLAtt fanaesu 



s (rf Ok. solar enipuont, I wnll coofioc myiwlf lo 
£11-;^ ibe oambcTj of die protatiCTWiioes whidt iluriag the cuittsei^lk 
- obser^ at>]D& tuic auauied ot txattAtA tbe tuaeht of ■', or 2', a! j, al 
4'. 2ad of 5'; fiuin Blud* OB bt taftrnd Uk degree of prubabilil; li 
be ^a^ned tt. the pralKiion cf phetOTac M trittun iht bilicuoi 
l^^ii- Taking -nto MC«aU a«)]r Itac pncubennccs obscncd in (k 
^eacra: Eg\ire of e>e sotor cii C Mwi efeiKg U jKj profiles, and leavnigoui 
tbe Moall jcu an<t [voabaaiices ofaoemd UM drawn tn ihc piirTal 
ti^rcrcs. ^ad in tbe nncalun or tncanplete profiles, tbe toLiI nmbtr 
oi sh^ protubersijoe* t& 7,449. 1 nnisi obaerwe, boivevn, thil itui 
number is cbuia^d by dtviding the cmapuund prolubcrancM ei \k 
iirge groups m:o the cUef coraponent groaps. and ihc pTulutwnncti 
cuBipi>i«i of one jets, inU) the chief clusters ol which ihcy conwL 

" Besides this :otal som we have 1.363 prolubeianccs which but 
^Mined oreiceeiJed the bcigfat off', wtuch numba may be divided n 
t\jj<i»s. according to tbeir height : — 

■■mbcr of tbe pratnbcnnccs . 

174 






mbcra, it 



it IS shown that ot above 100 prolubennccv 
md exceed ibc betgbt of 1' ; of above i/no, aUiui l' 
^M.et'd the bcigfai of ^ ■ aftd of more than lo^ooo, ub«ui47 
eieced the height of 3'. We must therefore crtniiitn 
»h!ch CKCCCil tbe grcal hcigbl of 4" as somethini 




THREE YEARS' WORK WITH THE NEW METHOD. 



time, 8-5. which was occupied by the intervening space in passing over 
■-the slii of the spectroscope. Allowing for the obhquiiy of the motion 
a the parallel of declination, the length of path piissed over by this ■ 
loud was more than 90,000 miles, and the velocity above 120 miles per 






laU tit: stoty of mAa 
!fis J u c ripti tM' is sonrid 
Be «» qaofec it (it atOM. 



bttaea h J f piK twelve aod two PJL 
I ■ii lnMi fainwMg- 

had bees T—iTrntrf** 

7; — qfw ^ rt tofc er j Mcg or hy J wy ctoiJ » 

"It hid iiJMiiiifdwil>»qThilcrhme' *«>« the |»*ctding noW- 
a Ipof, lav, faid-loakiac dood, mm very deasc or bnOiont, mn n 
s»)r wxy raurkkbie eaccM lor its liw. It wss made up mo«ilT<' 
fiLuoeoit ^artf boriBOOiaX and flootod abore the chramospbert "ii^ 
iia Lower Mif&«« u > be%tit of mxm: ■5/no miles, bat •» cxHiRKit^ 
U) it, U i* u*ually the cate, bj Ibtcc or four vettical caluauis brinhiB 
and more active t)un the rcsL Lodcj'n compares svch masm "■ 
a bajtyin grove. In Icngtb it measures 3' 4;*, and in ele\-aii(ni jboii 
]' to III u|>pcr surface— inai is, since at the sun's disi%nre 1* eqw'' 
4{o mittr* nearly, it was about 100.000 miles long by s-h^MO higb- 

"At II. JO, when 1 was called away for a few itiinutct, tbcrt *■* 
nu indication uf what was about to happen, except that oncd^ 

' Jouriittl 0/ Ihe FranHin /(»/(/tf/<r, November 1870. 
' Hritnii Jaanml nf Chemistry, November 1871. vol. vi. fi 



lEE YEAJiS' WORK WITH THE NEW METHOD. 



ting stems al the southern extremity of the cloud had grown 
irably brighter, and was curiously bent to one side ; and near 
le of another iii the northern end a little brilliant lump had 








1 pretend to accuracy of detail, except (he 
.(.-. ..g nearly exact. 



SOLAR PHVSICS. 



literally blown lo shreds by some inconceivable uprush Ironi beacaih. 
In place of ihe i|uict cloud I had left, the air, if I may use the c<prts- 
sion, was filled with flying lUbris -a mass of detached vertical fuiilonn 
lilamencs, each &Dm lo" to 30" long by z" or 3' wide, brighter ind 
closer together where the pillars had formerly stood, and lapidly 
ascending. 

" When I first looked some of them had already reached a bdgii 
of nearly 4" ( 100,000 miles), and while 1 watched them they rose «illi 
a motion almost perceptible to the eye, until in ten minulcs(IAi) 
the uppennoji were more ihaa 300,000 miles above the solar sutfice. 
This was ascertained by careful measurement ; the mean of ibm 
closely accordant determinations gat-e 7' 49" as the extreme altitwk 
attained, and I am particular in the statement because, as fu v 1 
know, chromoapheric matter (red-kyarogen in this case) has nem 
before been observed at an altitude exceeding s\ The velocity of 
ascent also, 166 miles per second, is considerably greater than any- 
thing hitherto recorded. A general idea of its appearance when ik 
filaments attained their greatest elevation may be obtained from 
Fig. 136. 




which fade " As the filaments rose they gradually faded away like a disWlviK 
as Ikty cloud, and at I.15 only a few nimy wisps, with some brighter stmnc- 
au-eaJ. low down near the chromosphere, remained lo mark the place. 





"But in the meanwhile the little ' thunder -head," before alludeJ '"■ 
had grown and developed wonderfully, into a mass of rolling anil *>"- 
changing flame, Id spenk according to appearances. First it ■'* 



THREE YEARS' WORK WITH THE NEIV METHOD. 



yoMHg'l 



crowded down, as it were, along the solar surface ;,laier it rose almost 
pyramidally 50,000 miles in height ; then its summit was drawn out 
into long filaments and threads which were most curiously rolled back- - 
wards and downwards, like the volutes of an Ionic capital; and 
finally il faded away, and by 2.30 had vanilhed like the other. Figs. ^;,^. 
137 and 133 show it in its full devclopmeni ; the former having been 
sketched at 1,40, and the latter at 1.55." 

We begin with a height of 200,000 miles, and get 
that entirely blown to nothing in a matter of a few 
minutes ! A very important point remarked by Professor 
Young in this prominence, is new. You may recollect 
I mentioned that the velocity of the solar winds, deter- 
mined by the changes of the wave-length, was some- 
thing on tile average like 120 miles a second — not an 




f»<3ur. but a second ; and that the ordinary velocity of 
^tt<^ uprush, was something like forty miles a second ; but 
tltis prominence was absolutely seen visibly by I'rofes- A /■miui- 
»ot Young, to rise with a velocity of 166 miles a second. Xti'J^/" 
^w this questions arise which I have not time to enter /«■ ifton-f. 
■«**o. but at all events I think we may take this observation 
**f T*rofessor Young's as entirely endorsing and dcpoetizing. 



THREE YEARS' WORK WITH THE NEW METHOD, 



403 



very nearly to the edge of the sun, and before it got 
quite to the edge of the sun, close on one side of it, I 
saw what I had never seen so decidedly before, a tremen- 
dous alteration of wave-length in the F line, — the F line 
being deflected first very violently to the violet, and then 
as violently to the red end. Also, in the same locality, 
I saw the F line broken into two parts — it was doubled. 
And what was going on while this was happening } A 
prominence, obviously with its root some distance from the 
limb, had gradually travelled beyond the limb ; in appear- 
ance it became very much more elevated, and seen, as 
it were, in perspective over the limb ; but what I saw 
first was very rapidly changed, in a way that would be 
explained by supposing that cyclones were being shot up 
into the solar air like bombs! the changes in the F 
line were so rapid and curious. I was not observing with 
an open slit, so I at once coined the term "motion forms," 
because the forms observed did not in any way represent 
the shape of the prominences. But the extreme velocity 
can be imagined from the great departure of those bright 
lines from the stable dark line F, seen below them (Fig. 
149); and not only that, but we can think out the explicit 
character of this prominence action. They were really 
in this case, as already stated, smoke rings thrown up by 
enormous circumsolar action. There are indications of 
enormous motion in one direction, and there are enormous 
motions in another. I watched the phenomenon for a con- 
siderable time, and then placing the slit tangentially over 
the root of the prominence which had travelled nearly 
to the limb, I saw that instead of one lozenge, as we had 
to start with, we really had three. 

Now, what does that mean ? Bearing in mind that in 
all that has been stated we have associated the thicken- 
ing or thinning of a line equally on both sides with an 
increase of pressure, you will see that in this lozenge 
we have a very fair indication of an actual uprush of 
the high-pressure hydrogen from the photosphere. And 

D D 2 



CHAP. 
XXV. 



Behaintntr 
of F. 



''Motion 
forms y 



A ioztngf 
indicatis 

high 
pressure. 



THREE YEARS' WORK WITH THE NEW METHOD, 405 



surface of the sun is so small that it requires magnifying, chap. 
so to speak, before it becomes apparent, and this magnifi- ^^^' 
cation we get at the edge of the sun, where a greater 
thickness of atmosphere is brought into play. 

Hence we should hold the faculae to be the higher 
cloud-domes, so to speak, of the photosphere. Since I 
last addressed you, they have been stated to be nothing 
but the permanent roots, so to speak, of prominences, and 
I shall have more to say about this by-and-by. But there 
is one point of great interest, namely, that the idea which 
has been started that the brightness of the faculae was 
due to the presence of bright prominences on the sun, and 
therefore associated with the presence of bright lines in 
the sun, has, I think, been entirely negatived, although I 
confess all the observers of these phenomena are not yet 
content to see in the brightness of the faculae merely a Brightness 
decrease in the general and selective absorption, which at ^^/^^^^ 
times is so great that in the spectrum of a facula you see decreased 
'very often very many Fraunhofer lines of the solar spectrum ^^^^P^**^^- 
disappear. The absence of continuous absorption, to 
which I strongly hold, has not sufficiently been brought 
forward among the evidence. Of course I do not affirm 
that bright lines are always absent, I only hold that re- 
duced continuous absorption is always present. The time 
has now arrived when the distribution of the faculae 
should be carefully studied, and I believe Father Secchi 
has already begun this branch of research. 

The Base of the Solar Atmosphere. 

Having thus dealt very briefly with the new observations 
and ideas connected with spots, prominences, and faculae, we 
are now in a position to consider the present state of our 
knowledge concerning what we may call the base of that part 
of the solar atmosphere above the upper level of the photo- 
sphere. You may recollect that in my last lecture I stated 
that Dr. Frankland and myself had come to the conclusion 



4o6 



SOLAR PHYSICS. 



CHAP. 
XXV. 



hirst 
tiolioits. 



that the existence of any such extensive corona as had been 
drawn by previous observers, — coronas extending beyond the 
sun two or three solar diameters, was extremely improbable, 
and our reasoning was based upon these facts — facts 
as they then appeared to everybody. First of all, that 
hydrogen seemed to be the only element normally present 
in those circumsolar regions^ then being examined with all 
the freshness of novelty, the strange yellow line D, being 
considered as a hydrogen line, which for some reason or 
other we could not detect in our terrestrial hydrogen. 
Another idea was that the pressure was extremely small; 
another fact to be borne in mind was that hydrogen was the 
lightest gas known to us — the element with the lea5t 
atomic weight, so that it seemed extremely improbable 
that anything could lie outside it ; and I told you by the 
way, and it shows you how careful one ought to be in all 
these things, that a certain line — that numbered 1474 on 
KirchhofFs scale — which had been seen by the Americans in 
the eclipse of 1869, was one of the most extraordinar)* 
indications we then had of the eruption of iron vapour 
to a considerable height in the prominences. Now, hat 
the progress which we have to record is truly cnonnoui 
In the first place we begin with a low chromosphere, lot 
pressure, and simple composition ; and I may say broadlr 
that we end with high chromosphere, high pressure, and mosfiMe 
complicated composition. In the first place, we are pcrfecth;-^ 
D^ditfstwt certain now that the line D3 has nothing in the worldtcz^^ 
/lydro^en. ^^ ^^'it\i hydrogen. On this head, in addition to the e\'idcflc»c3 
furnished by Professor Young, to which I have already--^* 
called your attention, and to other experiments of naycf^*^ 
and Dr. Frankland's, I will refer to our eclipse work in India.- •" 
which will at once convince you that the line D, has noduflg 
to do with the hydrogen. In Professor Respighi's ohsena- 
tions which I hope to have time to explain to you XDff^ 
fully,^ I will call your attention to the four arcs, one rei 
another yellow, another green, and another violet. 

' See p. 374, where these observations are discussed at leiigdL 



Enormoui 
progress. 



TJ^REE YEARS* WORK WITH THE NEW METHOD. 407 



These are the arcs seen by Professor Respighi, formed chap. 

by the prominences and chromosphere which was about to ^^^' 

be eclipsed. Now consider the three large rings, two 

of them mere continuations of two of the arcs, the one 

the ir^d, the other the blue, the green ring being an inter- 

pola.tion^-of which more presently. I wish to point out 

^hat if the yellow line were as much part and parcel of 

the hydrogen spectrum as the red line and the blue line, 

^c sI:^ould have had here a yellow ring as well as the red 

^^^Z» and the blue one ; but you see that the yellow ring 

has entirely disappeared, and the reason of that is that 

the tmydrogen absolutely physically present in the sun on 

^^ ^ ith of last December did not give us that yellow line ; 

and therefore, I think, we may look upon that as the coup 

^ S^^^^ace to the notion that the yellow line is due to 

"y^^^^Dgen in any solar condition whatever. 

^^^on after the first discovery of the materials of which 

"*^ mitvi stratum round the sun was composed, it was 

stat^^^ by Father Secchi that separating altogether the 

**y^**^gen from the photosphere was a region in which StcchCs 

^'^^^ was no hydrogen, but which gave us a continuous ^^^J!^.'*^'^ 

^^^^um. Now all the work which had been done by layer, 

^^^^""i^body, excepting Father Secchi, who had taken up 

^^ question, went entirely in favour of the notion of 

the perfect continuity of the solar atmosphere from the 

"^^^t point we could get at to the lowest point we 

^^^^ get at ; and therefore I objected strongly to this 

^"^"^mim, whether it gave us a continuous spectrum or 

^"^"^lier it did not, because I could see the lines of 

?y^ ^^ogen absolutely continuous up to the photosphere, and 

^^, ^*>e spots, so to speak, down to the bottom, because occa- 

J^'^^Uy in the spots you got hydrogen lines thickened, 

. .*^<^ugh that usually is not the case. Hence I objected to Objections. 

*^ interpolation, considering that what Father Secchi 

. ^^'^d a real continuous spectrum might probably have 

• ^*> an apparent one. As I remarked at the time, there 

Evidence of much reduced selective absorption, and 







I d«K to Oe pfaoCophen: 

i) Coapeetn 

■ thcfc, uid oy 

xltue- 

=:n3faer of Kks at Oe bae ttf tbe duo- 

c:.'j^^iicr£. 2::.i thgrefere Maneaged Ae — ■'m^ of Gamuts 

! ~-tT^ ij^ 1 tiL^r^'jrc omcsBco Ac pRSKnre n tliis f^MiL 

L^: cr:- '-jt cMatioa ooe or tvD of these obscr— 

; I 





TJfUEE YEARS' WORK WITH THE NEW METHOD. 409 



vas working with him. In the eclipse of 1871, the same chap. 

th/flgr- to a certain extent was made out, at Bekul in India _J^V 
wlier^ I was, by Captain Maclear, who was armed with a 
veryr fine spectroscope ; and the spectrum of the base of 
the c^hromosphere was very carefully watched, as it gra- 
dual! 3/ retreated, .so to speak, along the limb of the sun, as 
totals -^y came on. Under these conditions we certainly 
saw ^^ very large number indeed of solar lines, extremely 
faint^ and extremely short, but still bright, and not all com- 
ing ^^^^p to the same height. I may mention that all the 
Indis^si observers did not get out this fact; but still, 
takir^^ the evidence as it stands, I think that the Indian 
obsex- -nations, taken in conjunction with all the others, 
sho\^^ there is a real increase in the number of lines in these 
lowfcx- r^ions as this lower .stratum, which rests on the 
photi^i^sphere, is approaclied, and the hollows of which are 
douk^^^less filled with denser vapours still, for, of course, in 
"*c IxoUows and close to the exterior of the photosphere 
mus^ the reversal of the dark lines be effected. 



^ <:> we know anything more of the photosphere itself ? 

Notlrx ing certain, but I may here mention but very briefly 

*° *^^ ^a thrown out by Professor Zollner, that in the photo- Zoiifur's 

sph^-H-e there is a liquid layer some eight seconds below the ^^^nf- 

upp^x" niveau of the photosphere ; and he attributes to this 

^c ^lark appearance of the nuclei of spots, and looks upon 

It a^ the region where all the pressure is put on to the 

hydx-ogen, after escaping which it bursts out in the tre- 

meadous way Professor Young and others have seen. But, 

unfojTtunately, if you examine Professor Zollner's paper 

^^^'^'villy^ you will find that he has taken necessarily as 

data, in j^jg calculations, a great many of the facts which 

profc>^t^jy j^Q^ ^;u require a certain amount of recon- 

sider^tion. For instance, he has taken the pressure at the 

V ^^ ^^ photosphere as extremely low. Now I myself 

s ouid not now be prepared to admit that extremely low 

pressure. 



SOLAR PHYSfCS. 



Of course the region we are now considering may not 
only be regarded as the base of the prominences, but a 
tliat ill wliicli the spot phenomena, if not quite arc very 
nearly carried on ; and it may happen — I throw this out at a 
suL^i^restion — that we shall find that as the spectra of the spots 
changi: from cycle to cycle — I have already told you the 
extreme difference in the Spectrum of the sun-spots, as seta 
at different times — we may find that the same cause which a 
at work in raising and lowering the niveau of the spots, and 
in producing the welling-up of the magnesium and iron, may 
have something to do with this extreme complexity of the 
chromosphere which has lately been determined, and il 
may be found that it will change from time to time. I 
may here refer to a photograph which I was able to talct 
a few days ago of a part of the spectrum, which includes 
the two lines H — H' and H'; — and although I do not at 
all state this connection as a final one, I think you wiUkc 
1 tliat if we can associate any of the changes in the thicken- 
ing of tht lines with any of the changes in any way 




THREE YEARS' WORK WITH THE NEW METHOD. 



411 



these two very dark lines on the photograph, three only 
are recorded where the photograph shows 43, you will 
recognize at once the importance of the new kind of record, 
in which if there is a change in a series taken, say every 
three months, such change cannot escape detection. 



CHAP. 
XXV. 



The Limit of the Solar Atmosphere. 



We will next carry our inquiries to a somewhat higher 
r^^on — to that region of the sun's atmosphere, namely, 
which lies above those to which I then referred — the 
region of the corona. It will not take me very long to 
convince you that in the matter of our knowledge of the 
limit as well as of the base of the solar atmosphere, our pro- 
gress since I last addressed you has been enormous. You 
may recollect that I told you some three years ago that 
the Astronomer Royal, and a German Astronomer, who 
carries almost equal weight in his own country — I refer 
to Madler — after they had both of them viewed the 
eclipse of i860, and not only had seen the eclipse them- 
selves, but had had an opportunity which they had 
largely availed themselves of, of studying all the obser- 
vations made by the other astronomers who were there 
assembled, they, I say, came to the conclusion that the 
corona was a compound phenomena, part of it being un- 
doubtedly solar, as everybody, of course, was willing to 
^dmit, seeing that part of it had been seen time out of mind 
even before the sun was eclipsed ; but that part of it also 
^as in their estimation not solar. Up to the eclipse of i860, 
^^ had not that truly unmistakeable record of the corona 
^hich we have now. For instance here, again, is the picture 
^* the corona seen in the eclipse of 1858, which I brought 
^ your notice three years ago with the remark that one 
P^*^ of it was undoubtedly solar, but that about certain 
^^Aer parts there was still some considerable doubt. I 
^^^t showed you the corona visible round the eclipsed sun 



Equal 

progress on 

this point. 



The 

corona a 

compound 

pheno' 

minon. 



Coronets of 

1858 and 

i860. 



SOLAR PHYSICS. 



in the j*ear 1869, in America. In this we saw somctluDg 1 
did'ereat ; and here, in fact, has been the difficulty Jn all 
these obsen-adons, that whether we deal with the flux of I 
time, from ecUpse to eclipse, or whether we deal merely 
■ftirh those differences which arise from change of stition, > 
we get such enormous changes in the pictures ^vhich are 1 
presented to us, now by one eclipse, now by another, now I 
by one observer, and now by another, of the same eclipse. I 
From these causes the thing is very much more difficult 
thin the ordmar>- beholder would imagine that it could be. | 
la the drawings made by Plantamour in lS60v we find 
■ another line of evidence, namely, that actual changes in the 
corona were observed to take place during the eclipse. 
Vo-j may recollect that I told you that the result wbick 
Dr. Frankiand and m>-5elf, in the laboratory', had arrived at, 1 
seemed to entirely agree with this line of thoughL Wc ^ 
did not quite see how with the low pressure which »x bad | 
•ieterinined, on the assumption of a simple composition of 
the chr^irr.c >phere. there could be any verj' large coron* I 




THREE YEARS' WORK WITH THE NEW METHOD, 



413 



and Spain, in December 1870; another to India, in 1871. 
Unfortunately, as most of you know, the weather in 1 870 
was very bad indeed. The important fact determined in 
1870 by Professor Young was connected with the line 1474, 
the American observations of 1869 being endorsed and 
extended. It is the study of this line which has taught us 
the existence of some unknown element extending further 
from the photosphere even than hydrogen. That it is 
not iron, as was at first supposed, I have lately determined 
in a series of researches now about to be laid before the 
Royal Society. 

At the same time that this line was observed to extend 
to a distance of 20' from the sun, the lines of hydrogen were 
observed eight minutes above the sun ; this was supposed to 
be due to reflection, or some similar cause. A comparison 
of photographs, taken in Sicily and Spain, seemed also 
to indicate in the apparent boundary of the corona, dark 
spaces called rifts, which have been acknowledged to be 
identical in the two photographs : this part of the corona 
was therefore solar. Now in the observations of the last 
eclipse we have determined that these rifts really represent, 
as it were, indentations into the solar atmosphere, reaching 
at times to a considerable distance. Moreover by a new 
method of research we were able to prove that hydrogen 
tcally exists to a height of some seven or eight minutes 
at least above the sun, far above the vividly incandescent hy- 
drogen which we can see by the new method without waiting 
'Or an eclipse ; so that this after all is only the base, a small 
^O layer at the base, of an enormous envelope of hydrogen, 
Pt>bably on the average twelve minutes high, according to 
^^ photograph, if we accept the photograph as belonging 
^ tJhe hydrogen as well as to any other materials. And this 
'^^^cs an important question : while in the photograph the 
^ges of the corona are seen quite jagged, the limit of the 
^f ona as indicated by the spectroscope was perfectly re- 
S^lar. There, again, is great food for thought, and it is a 
Question about which we shall be able to speak with much 



CHAP. 
XXV. 



Hydrogen 

higher 
than was 
imagined. 



414 SOLAR PHYSICS. 

CHAP, greater certainty than it is possible to speak now, after the 
^^^_ _ experience of two or three more ech'pses ; and we shaD 
probably find it is associated with phenomena connected 
with a possible reflection of solar light by the materials 
of which the solar atmosphere is composed. I can oolf 
very briefly allude to this point, but on it the verdict I 
think, is now flnal, although science has been oscillatii; 
first one way and then another now for a good manyyeau 
on the subject. Dr. Janssen, the eminent French obscrw. 
who was observing in India not far from one of ourstatkn 
was fortunate enough to detect in the spectrum of tk 
corona not only bright lines, as we all of us had done, but 
dark lines which could only have been due to sun l^b 
reflected by something in the corona, and the polariscopc 
also indicated much reflection. 
Detailed Now then, accepting these photographs, and acccptin 
study, these spectroscopic and polariscopic observations as givi< 
us some just notion of the sun's outer atmosphere, let « 
look at it a little more in detail. In the first place « 
have, let us say, twelve minutes high all round the sun a 
boundary more or less jagged, more or less circular, as 
you will — that will depend upon whether you accept tk 
photograph as representing the truth of nature, or tk 
spectroscope. We have then an atmosphere twelve mimtf 
hi^h, the outer portion being composed of something aW 
which we know absolutely nothing whatever, except ttt 
it gives us a line in the green which independent research 
have shown is not an iron line. Let us call thatx 
— you will see why presently. Then we have also roiP 
the sun, at a height of, let us say, eight minutes, h)'droS* 
But here we must pause somewhat, and divide our bydff' 
gen into layers. We may undoubtedly divide it intot* 
strata. Here you see we approach the region and sofl*" 
what, therefore, the language of geology; and probal'F 
we ought to divide it into three strata, but let us be c<fr 
tent with two. We have the hydrogen sub-incandescrf* 
which we cannot see by the new method, and the hyi^ 



THREE YEARS' WORK WITH THE NEW METHOD, 415 



gen incandescent which we can, as its temperature is chap. 

sufficiently great to make it excessively brilliant, so that __: _ 

we see it without waiting for an eclipse at all. Let us 
place this two minutes high. Below this, then, we have 
the yellow line giving substance, which is mixed up with 
the lower hydrogen ; which is very rarely seen above 
the hydrogen, but which is often seen low down, in such 
2 way that we are perfectly satisfied it is not hydrogen- 
Let us call this x. This probably may be placed one 
miaute high. Then, from the observations on the base of Succession 
the solar atmosphere, we have magnesium, and sodium, Yn^tkeTun 
then barium and nickel, then iron, and a host of other 
substances, and travelling down from x', which gives us 
the 1474 line and exists at the extremest, the most utterly 
distant, parts of the corona, right down through the solai 
atmosphere to the bottom of the deepest spot, we shall 
pass very much through the different substances in this 
order. Beginning with the 1474 element, we pass through 
the sub-incandescent hydrogen ; deeper still we get to the 
incandescent hydrogen ; then we go through the D, ele- 
°^€«it; then we get into regions where the lines are 
S^i^erally mixed rather more together, but from which 
"^^^grnesium and sodium are generally ejected more fre- 
^''^ntly and higher than any other material ; then we 
t^ into the more doubtful zone of barium and nickel, 
sometimes sodium being thrown up, sometimes barium, 
so^ietimes nickel; and then we come lower down into 
^''^"^t may be called, so far as we shall ever be able to 
^'^v^^stigate the sun, the very bowels of our central orb, 
wh^yg we are certain to get iron, and we may get many 
oth^r materials. 

I will finish the picture of the exterior portion of the structure 
^^^T atmosphere by referring to its structure. On the last ^^^'^«''- 
^^^a.sion I showed some photographs of prominences which 
^ Had sketched, and I told you that the region of the 
Prominences in the sun, reminded one almost, now, of a 
^^*"est of banyan trees, and again, the chromosphere put 



SOLAM ^BYStCS. 



~^ :>i am Eaglsb hedge-row, the promi- 
-■t 1 'iwjjMl dnts. Tbat definition stiU. 



I 




•xtttt^ good. The structure is fila- 
i iisffaaace is not going on. instead ot 
■~t ictnn, in vlndi the promintMict^s ar^ 
- -n get a nebaloas appearance, '^■o-f 
■.t^^ of Ac corona may be detincij it* 
I examined the corona witl^ 
, sad andd' first-rate atmo5pberi<= 
I n latse arc of it, and there 
-<rxa, vladi 1 saw. which could not hav-^ 
rfrx-i .j a»l prnduDcnces; but in one pari o£' 
zr- ;cr;c.i rie '..-; Kara, 9D to Speak, predominated, an «J 
* t^oO'^ pi-t ii::: cofoaal masses got more and more t.<3 
"n^sf^zx-i ^:^^cse srr^^e tioi^loaKrations which we ^ee not 
.-c_y 13 ioc^e int ire pni^ncsoesv bat also in some of iHc 
?ir^ju^ 3c£h;^ io mach so that although I was not very 
— ^^ -xrneii. I f:^ did ^*'^j"". peihaps more loudly thaJi 
I •ixc hire dooe- that the corona looked like the ncbul^a 
"f Or>>c ", isd ii>ere was one c^pcdal part of the coroti-a 
m'ixt'z ^e^^^f■e■i ce moat ri^-idly, even amongst all th« 
jx-:;r:e=; ot d:,-5?e two minutes, of that exquisite dra»*'' 
-; ^iln by tie late Sir John Heischel of the nebul-a 
Arj,r, rassmg then from one part of ih 




f^JtEE YEARS' WORK WITH THE NEW METHOD. 417 



as if hydrogen alone or almost alone extended chap. 
ve the photosphere, and the name was given to denote ^^^' 
bright line region in which a new world of phenomena 
I daily revealed to us by the new method. We now 
)w that out of the reach of the new method there is a 
;ion of cooler hydrogen and something else — what that 
? IS we do not yet know — but we have now to take it 
o consideration. I proposed, therefore, that the term 
otosphere should hold for all the solar material outside 
6 chromosphere, as its continuity was undoubted, leaving 
c word corona for the mixed phenomenon, which we see 
icn we can see it — that is, during total eclipses. But 
thjanssen and Respighi have considered this nomen- Rnnscd 
ture inadequate, and propose to restrict the term chro- nomm- 

, 1 1 !• .Mil 1 clature, 

•Sphere to the solar surroundmgs visible by the new 
thod, naming the exterior portion atmosphere coronale or 
omosphhe exterienre. I willingly yield, with the remark 
t the last of these terms seems to me to be the better. 
Cepting this nomenclature, then, we have — 

{Exterior. 
Interior. 
( Photosphere 

Vhat, then, is the interior chromosphere ? It is un interior 
ibtedly a region which as a rule bounds the convection ^^I^^^f' 
rents of the sun ; it is a region where there is a sharp 
ilc in temperature: its hairy or cloudy outline probably 
ending somewhat upon the upper incandescent air. 



The Movements of the Solar Atmosphere. 

have now to pass as rapidly as may be to another 
r^ase of our knowledge — as great as that we have 
^ined from those two eclipse expeditions, — I refer to 
ac magnificent work done in the clear sky of Italy by 
lessor Respighi, which has taught us more about the 
Krements of the sun*s atmosphere, by means of the new 

e e 



4i8 SOLAR PHYSICS. 



CHAT. method, and in a couple of short years, than we could pro- 
^''^- bably have learnt without it in thousands of centuries. 

What do we know already about the sun's atmosphere? 
In what has already been done we are limited espe- 
cially to the base of that atmosphere. We have been 
limited, so far as a complete and careful study of it is con- 
cerned, to the phenomena of the spots. W^e may say 
roughly that the salient phenomena of the spots are som^ 
thing like these : sometimes we have a great many spots 
in the sun, sometimes we have very few : the time from 
a minimum to a minimum or from a maximum to a 
maximum is somewhat over eleven years, but there are 
longer periods still. 
spot zones. But this is not all that astronomers have been able to 
tell us about the distribution of the spots. The work of 
seven long years is recorded in Mr. Carringtons book on 
the sun, and his plates show very clearly a fact that was 
gathered by the first discoverers of the spots, namely, ih^ 
they affect certain regions. Let us draw a diagram of 
sun, and represent on it the solar equator and the parall^ 
of thirty north and south latitude. As the time of u" -^ 
mum sun spots — when there are no spots or very \t\ 
the sun — passes by, what happens } We b^n to sec 
spots putting in an appearance at about latitude iM", 
north, and thirty south. As the maximum is more ne*. 
reached they gradually approach the equator ; the ^. 
widens, and the gradual increase in the range of the* ^j 
becomes very apparent : the spot zone, that is to say.^^-is 
wider as it gets nearer the equator. We have their ^.«". 
two spot zones — one to the north, and the other t^^ 
south of the equator ; the amount of spot freqr^^^BjSB 
increases rapidly, and at the same time the zone in ' wi 
the spots mostly appear varies considerably. Proczzoiajl 
future investigation will show that these zones ai ?^ 
absolutely symmetrical with the sun s equator. Here — in 
we have roughly the outcome of a great many y^^^^ 
work on the sun's spots — those sun-.spots lying at the «f 




THREE YEARS' WORK WITH THE NEW METHOD. 419 



the solar atmosphere. Now it is clear to you that if chap. 

V V V 

J could get anything higher up in the solar atmosphere, !_ 

here the pressure must be much less, and where we 
ow that the changes are much more rapid) to render 
ident to us exactly what is going on there ; and if from 
ese records we found that there is a method in all the 
parent irregularity ; it is obvious that we should have 

this way a much greater chance of being able to get 
dily into the secrets of these solar regions than we 
ould in any other way. If in the region of the spots 

is extraordinary for a spot to change very much from 
ay to day, and if in the region of the prominences it 
^ extraordinary if they do not change from hour to hour, 
t is evident that a very little labour will go a great way 
owards showing us first of all whether there is a law, or 
Aether there is not a law, which regulates the apparent 
'Wgularity with which the prominences make their appear- 
'^ce; and if there be a law, letting us know what that law 

^ow, pre-eminent amongst the men who have worked at RespighVs 

^ branch of research, Professor Respighi must be 

't'oned. I shall give you some idea of his wonderful 

^uity when I tell you that since the discovery of this 

^'^cthod, he has already mapped more than 8,000 pro- 

'^^^^s, and when we come, as some of you do, to know 

^*3^ ^vhat that means ; that every day, or sometimes two 

""^^ times a day, you have to bring the sun's image on 

^ ^lit-plate of your spectroscope, and then carefully go 

the whole limb of the sun bit by bit, carefully re- 

*^S the position and form of all the prominences, be 

^ig or be they little ; most carefully measuring their 

at the same time ; it will be perfectly clear to you 

labour of this kind is one of enormous magnitude. 

here a diagram which will show you the way in 

^ such observations are recorded. The prominences 

^ "^ mapped on a circle, representing the profile of the 

» '^ith the north, south, east, and west points and the 

K K 2 





W SOLAR PHYSICS. ^^^| 


r 


position of the poles of rotation ; or they may be O^^IH 
along a line in each case beginning at the nortiwraj 
Joint of the sun, inserting the prominences at the proper 

degree, and then afterwards noting tiic exact (wsilion oi , 


I 




f 








■1 


I 




1 






■ 


BB 
















1 








■1 




II 


i 










1 




1 








^^^B 


i 










1 










^9 


i 












^B 












■ 








^H 


? 














1 








M 














K 








Bii 




















H 




1 














I 


BH 








HHH 


B 


the sun's equator and pules. In tliis graplnc way tk 1 
history of every prominence is written down from divt 1 
day. if the weather be fine. You have now an idcj c" M 
the enormous work which has been done by Vf«k/^^k 



THREE YEARS' WORK WITH THE NEW METHOD. 421 

Respighi. And what then is the result of Professor <:^"^*J'- 

Respighi's labours ? Of the 7,449 prominences which he 

observed from October 26, 1869, to the 5th of May of the 
present year — he found that 1,330 are below one minute 
high, 1,150 between one minute and two minutes, and so 
he goes on till he finds only five exceeding five minutes 
in height. 

Then with respect to the frequency with which these Piyv-'»(y 
1 . ™« . r af firomi' 

promtnences make tneir appearance. Ihis part of our nmcti. 
knowledge is to be got from Professor Respighi's memoirs 
in two different ways. We have first of all the numbers 
of the prominences taken tt^ether, big and little ; and we 
learn from these wltat has happened with reference to the 
whole number of prominences visible. But in addition to 
this he had sorted out the more important prominences, 
the larger, fundamental, ones ; and we have the information 
tabulated again with respect to them. In these diagrams ' 
I have very roughly, but I hope sufficiently, shown you 
by means of zones, what Professor Respighi has found 
nrith reference to all the prominences on the sun's disc. 
His observations began in 1S69. I will read you the 
exact periods, Tiie first began October 26th, 1869, and 
extended to the 30th of April 1870. The second period was 
from the ist of May to the 30th of October 1870; the 
third period is from the 1st of November 1870, to the 
30th of April 1871 ; and the fourth period is from the ist 
of May 1871, to the ist of September 1871. /',.<mi. 

Now what makes these diagrams so e.'ctremely impor- "^^" 
tant is the fact, namely, that they show what is going on 
in the prominences as we approach the maximum period 
of sun-spots ; we get the prominence story while the sun- 
_p. jts are working to their maximum. Now, what is that 
stor>' ? J have before told you that the rise of the 
sun-spot curve, is very much more rapid than its fall. 
Therefore, in May 1869, we were further from the siui- 

■ It has been fuund Impo^siblL- to j^ivc lUc^u (liii;iraiii!> hiTc. but 
they are not cssenii.il. 



presenting a Urge number of prommenc^ 
away towards the two poles. The places whc 
no prominences arc the equatorial regions a 
poles, We pass on to the next period, and o 
a change in a few months ! The equator here 
blocked up with prominences, The well-defir 
have disappeared ; there are no longer promii 
there is an enormous prominence belt. I sho 
this does not represent the map of the plienomei 
accuracy, it is simply a rough sketch of what h 
1 have put the facts in their strongest form. In ' 
hemisphere there is a distinct massing of | 
which we do not find in the southern hemispl' 
same time both the poles are free from promir 
now approach another period — a period just 
maximum of sun-spots. Here we find the c< 
again, and with the open equator, as before, we ] 
of the symmetry of the prominence zones ; o 
of two definite zones, as we had in the first peri 
four, and they are gradually creeping toward 
Itere again, at the period of maximum of sun-: 
the equator closed again, as it was before, and 
the same lob-sidedness, so to speak, for there 
niiim to the south not represented at the north 
equator, in exactly the same way as when i 
was closed before, there was a maximumvfl 
side of the equator which was not representaS 



THREE YEARS' WORK WITH THE NEW METHOD. 423 



in the solar atmosphere, and make us acquainted nlore chap. 

accurately with the various cycles of solar activity. You all ' 

here are so interested in the movements of our own atmo- Connection 
sphere, that I need not remind you that year by year, solarTnd 
one might almost say day by day, the movements of our terrestrial 
own atmosphere, from a planetary point of view, are ^/^^" 
necessarily intimately locked up with the sun's activity ; 
so that the only proper way of studying meteorology is 
to begin, so to speak, at the fountain head, and study 
the sun. Mr. Meldrum, a distinguished meteorologist — 
who lives not in the temperate zones of the earth, where 
the meteorological conditions are irregular, but in the 
torrid zone, where regular meteorological phenomena, 
and among them cyclones, abound — tells us that it is 
»o longer correct to merely associate cyclones with the 
tropics. He tells us that the whole question of cyclones 
is a question of solar activity ; and that if we write down 
in one column the number of cyclones in any given year, 
and in another column the number of sun-spots in any 
given year, that there will be a strict relation between them ; 
niany sun-spots, many hurricanes ; few sun-spots, few hurri- 
canes. Only this morning I have received a letter from Dr. 
Stewart, who tells me that Mr. Meldrum has since found 
^^t what is true of the storms which devastate the Indian 
^^an, is true of the storms which devastate the West 
Indies; and on referring to the storms of the Indian 
^^^^an, Mr. Meldrum points out that at those years where 
^^ have been quietly mapping the sun spot maxima, the 
^""bours were filled with wrecks, vessels coming in disabled 
TOin every part of the great Indian Ocean. Now that surely 
'* Something worth considering, because if we can manage 
S^t at these things, to associate them in some way with 
^*^r activity, so that there can be no mistake about it, 
^ power of prediction — that power which would be the 
^st useful one in meteorology, if we could only get at it — 
^Uld be within our grasp. 



Tim METEOROf.OGY OF THE FUTURE. 



It would be a curious inquiry which we commend to thw 
learned in statistics, to determine how many millions p1 
observations have been made in the British Isles on d'V 
and wet bulb thermometers, on barometers, and on other 
niL'tcorological instruments. It would be a still maw 
curinua inquiry, seeing that the infinite industry displajtd 
in these obM-rvations shows that the importance "f the 
-Study of MLtfiTulo^y is universally conceded, to dfttniiin<; 




lETEOROLOGY OF THE FUTURE. 425 



vc manner, lay hold of, study it, record it, chap. 
: means. If there is no cycle, then despair ^^^'' 
>u will, but yet plant firmly your science on 
s, as Dr. Balfour Stewart long ago suggested, 
nfinite detriment of English science, he left 
^cal Observatory at Kew ; and having got 
s this, wait for results. In the absence of 
;, statements of what is happening to a 
> in vacuo, or its companion exposed to the 
earch purposes, work of the tenth order of 

ing to hunt down is a cycle. Now it may be 
re anywhere on earth a weather cycle ? but 
sks this question will at once answer it him- 
tion would certainly suggest the trade winds 
, which are short-period cycles. But is there 
; than this } 

preparing to go to India last year to observe CycU in 
In Ferguson, the able editor of the Ceylon ^O'^^"- 
happened to be in London, was good enough 
mough to us all afterwards, and the Eclipse 

1 87 1 have much to thank him for) to give 
able local information about the time of the 
the monsoons broke up in the island. Nor 
e added that everybody in Ceylon recognized 
ut thirteen years or so in the intensity of the 
t the rainfall and cloudy weather were more 
:hirteen years or so. This of course set one 
solar matters thinking, and I said to him, 
sure the cycle recurs every thirteen years ; 
it is not every eleven years .^" adding as a 
e sun-spot period was one of eleven years 
s, and that in the regular weather of the 
where, this should come out. 
sation Mr. Ferguson thought fit to reproduce 
Observer f and I have now lying before me a 
L number of that paper I saw in India (un- 



4 



aransE^- c £ =i£ H» mkIi. fcr hoA date aad writer') 

farcs (*> Locfcja sum ii^ 

KMd nas ivf' nr sit itm 

-:— .— ~**^* *' ""^ tier fcni lie 

— ^te^™i*^dfcrf AiKf or ttim-thnc ji** 
r v rflKiLHa qtikK. k« 1 matt rrcBBK hot 

ffWBi t^ fcr tWre nqr fee H 
qi s enad cydc if thntr « ] 




aif Ac doren-ycar penorf 

wUj-abo a li^er ooe stiK 

I> the pnas of worii dnC 

to Ei^Und, afta- nr 

pmvuteil from takis; 

upon this BKist 

evidence on the 

facte 




THE METEOROLOGY OF THE FUTURE. 



427 



ifall a fair test of the existence of a periodicity of 
lones in the Indian Ocean it would be necessary to 
»w the annual rainfall over the same area for the same 
grth of time. If such rainfall had no periodicity, we 
uld have reason to -doubt a cyclone-periodicity ; but if 
re was a similar rain-periodicity, it would, so far, be a 
lirmation of a cyclone-periodicity." 
Accordingly, as it is impossible to determine the rainfall 
T the ocean, the law of the cyclones of which has been 
Proximately determined, there remains but one course 
:n, to observe the rainfall on the nearest points of land. 
is is as follows for the above-named stations: — 



CHAP. 
XXVI. 



BRISBANE. 


ADELAIDE. 


PORT 


LOUIS. 


Year*. 


Rainfall 


Years. 


Rainfall. Years. 


Rainfall. 




Inches. 


1839 
1840 

184I 

1842 

1843 
1844 

1845 
1846 

1847 
1848 
1849 
1850 
1851 
1852 


Inches. 

19*840 

24-107 

17*956 
20*318 

17*192 
16*878 
18830 
26*885 
27*613 

«9*735 
25*444 
19*274 

30633 
27*340 




Inches. 






1853 


26*295 1 


.853 


39*829 






1854 


15*346 


1854 


39*435 






1855 


23*145 ] 


[855 


42*665 






1856 


24*921 1 


1856 


46*230 






1857 


21*156 1 


■857 


43*445 






1858 


21*522 1 


1858 


35506 






1859 


14*842 1 


1859 


56875 


i860 


5463 


i860 


19*670 


i860 


• 45*166 


1861 


69*44 






[861 


68*397 


1862 


28*27 






1862 


28-397 


1863 


68*82 






1863 


33420 


1864 


47*00 






1864 


24147 


1865 


24*11 






1865 


44730 


1866 


37*24 






1866 


20*571 


1867 


6ro4 






1867 


35*970 


1868 


35*98 






1868 


64-180 


1869 


54*36 






1869 


54*575 


1870 


7906 






1870 


45*575 


i«7' 


45*45 






1 87 1 


41*910 



SOLAR PHYSrcS. 



Now, we 
and maxim 



low, to start with, that the years of 
1 sun-spot frequency were as follows :• 



and Mr. Meldrum has shown that these years were alio 
those of minimum and maximum cyclone frequency. Lcl 
ua begin by examining the Port Louis Observatiwu, 
embracing nineteen years (1853 — 1S71). 

Taking the rainfall in each minimum and maxintn 
epochal year, and in one year on each side of it, Mr 
Meldrum gets — 




T^HE METEOROLOGY OF THE FUTURE. 



429 



^» again, a similar result is shown. It is not so well- 
-^ as the former one, partly owing, Mr. Meldrum 
•^ts, to the rain-gauge having been removed in 1866 
^^mporary Observatory, where the rainfall was pro- 
^y somewhat greater. 

So far, then, as the Port Louis observations enable us to judge, it 
f be said that during the last twenty years there has been a rain- 
■pcriodicity corresponding with the cyclone-periodicity in the Indian 
5^ south of the Equator. 

*his may be considered as confirmatory of the correctness of the 
'ofte period ; for if the rainfall at one station shows a corresponding 
^icity, much more should a mean of the rainfall at many stations 
in the whole cyclonic area do so." 

If. Meldrum next passes on to the Australian observa- 
3, remarking that, although Adelaide and Brisbane are 
ng way outside the area for which the cyclone period 
determined, there also the rainfall tables seem to 
It to a similar periodicity. 
he Adelaide twenty-two years' observations give : — 



CHAP. 
XXVI. 



Min. 



Max. 



Min. 




y taking five-years per 



Rainfall. 

17192) 
16-878 \ 
18-830 ) 

27-613) 

1973s \ 
25*444) 

23*145 
24*921 

2 1 I 56 



Total Rainfall. 
52900 

72-792 

69222 



Minimum' 
Maximum 
Minimum* 



ods we get : — 

- 100076 inches. 
= 118-951 „ 
« 106*090 „ 



Adelmdf, 



^c next come to twelve years' observations at Brisbane, Bnshme. 
vvhich science is indebted to Mr. Edmund McDonnell, 
iparing them with the Mauritius observations for the 
e period, we cannot but be struck with a resemblance, 
rh comes out still more forcibly when we take three- 
's periods, thus : — 



THE METEOROLOGY OF THE FUTURE, 



431 



:e Mr. Mcldrum s results have reached me, I have 
the Cape and Madras rainfall, to see if the same 
is to be got from them, and with the following 






Max. 

Min. 

Max. 

Min. 
Max. 



{ 
{ 



Cape. 
1847 
1848 
1849 






22*4 
23*2 
230 


1854 

1855 . 
1856 






20'0 

24*5 

19-4 


1859 . 

i860 

1861 






367 

29*1 
25*4 


1866 






19*2 


1867 
1868 






22*9 

19-9 


1869 
1870 






32*3 

28-0 



Inches. 



68-6 



639 



91*2 



62*0 



62*'^ For two yearf 
^ only. 



m the Madras observations at my disposal only one 
lum and one minimum can be given : — 



Min. 



Max. 



Cape. 




1843 . 
1844 

1845 . . 


41*0 
. . 450 

39*o 


1847 
1848 

1849 


8ro 
40*0 
54*o 



Inches. 



125 



175 



ely here is evidence enough, evidence which should 
ger allow us to deceive ourselves as to the present 
>f meteorology. A most important cycle has been 
cred, analogous in most respects to the Saros dis- 
"A by the astronomers of old. Indeed, in more 
±s than one, may the eleven-yearly period be called 
iros of meteorology, and as the astronomers of old 
profoundly ignorant of the true cause of the Saros 
I, so the meteorologists of the present day are pro- 
ly ignorant of the true nature of the connection 
!en the sun and the earth, 
lat, therefore, is necessary in order to discover the 



CHAP. 
XXVI. 

Cape, 



Madras, 



c:>t3c^ IT-- r> 



: TTss acx^^ ? Two tfaiags aic Dccessary. and 

s£ 1= -3; fast piaoe. we must obtain an 

c t^i^z ''hifoafteMsof the son, and secondly. 

_^ ^- iir-rzle kaovicdgc of the currmu of 

~~;' :(' i1k9C deBuads the uoited ctii>nj 

err i-r^ i'T^iPmk amaifias, and the second c4 

•i oe p :^->«t «f BW le orology as a physioil 

I of weather statistics 

id m spite of the Mrs. 

; to pre\-ent thi*. thc>- 

^—^i ^:1A3 Ibxc a Scioice of Meieorologj 

3 bajTr— Ac Meteorology of the Future. 



1- — Tb; jeiz^ 1:- sbid irfcrence has been madt- 
r=» rv Mr. T%-i^. as I kara from the CaiamiM Ob- 
-.< \i=:=3^ yj 1*7^ which oontains more facts 
iw -"^ sii-^ej::. Mr. Tytfcr la>-s great stress on the 
f ^^.'-r rrirr. }£ai5. pointing out that \isitatiuas 
ic5 of Ccylsa and \-ast landslips 




PART II. 

COMMUNICATIONS TO THE ROYAL SOCIETY OF LONDON 
AND TO THE PARIS ACADEMY OF SCIENCES. 



F F 



L^OBSERVATORY WORK. 



First Paper. 

^troscopic Observations of tJie Sun, No. I. — By J. N. 
.OCKYER, F.R.A.S. Communicated by Dr. Sharpey, 
ec R. S. Received October ii, 1866.^ 

E two most recent theories dealing with the physical first 
stitution of the sun are due to M. Faye and to Messrs. ^'^^^^' 
la Rue, Balfour Stewart, and Loewy. The chief point xke 

lifference in these two theories is the explanation given f^^*^^^ of 

aach of the phenomena of sun-spots. ^iL^^' 

*hus, according: to M. Faye,^ the interior of the sun J^^f^^,^' 

. , /rill.' De la Rue^ 

nebulous gaseous mass of feeble radiating power, at a steioart 
perature of dissociation ; the photosphere is, on the ^^^^oewy. 
ir hand, of a high radiating power, and at a temperature 
ciently low to permit of chemical action. In a sun- 
: we see the interior nebulous mass through an opening 
he photosphere, caused by an upward current, and the 
spot is black, by reason of the feeble radiating power 
he nebulous mass. 

1 the theory held by Messrs. De la Rue, Stewart, and 
%vy,* the appearances connected with sun-spots are re- 
id to the effects, cooling and absorptive, of an inrush, 

^roc. RS. vol. XV. p. 256. 

"^omptes Rendus, vol. Ix. pp. 89 — 138. See Chaps. 4 and 5. 

' Researches on Solar Physics." Printed for private circulation. 

or and Francis, 1865. Sec Chap. 6. 

F F 2 



SOLAR PHYSICS. 



Evidemc 



or de5Ct;ndi[ig current, of the sun's atmosphere, «4uch h 
known to be colder than the photosphere. 

In Jime 1865 I communicated to the Royal Astronomi- 
cal Society' some observations (referred to by the authors 
last uamcdj wliich had led me independently to the sann: 
conclusion as the one announced by them. The observa- 
tions indicated tiiat, instead of a Spot being caused by an 
upwaril current, it is caused by a downward one. and thai 
the results, or, at all events, the concomitants of the down- 
ward current are a dimming and possible vaporization of the 
cloud-masses carried down. I was led to hold that the 
current had a downward direction by the fact that one of 
the cloud-masses observed passed in succession, in the spsw 
of about tH-o hours, through the various orders of bright- 
ness exhibited hy faatla, general surface, and /xrnumdra. 

On March 4th of the present year I commenced a spec- 
troscopic observation of sun-spots, with a view of endeavour- 
ing to test the two rival theories, and especially of following 
up the observations before alluded to. 




OBSERVATORY WORK, 437 



vations. Hence I should have hesitated still longer to lay first 
them before the Royal Society had not M. Faye again ^^'*^'^- 



recently called attention to the subject. spfctre- 

On turning the telescope and spectrum-apparatus, driven ^^ 
by clock-work, on to the sun at the date mentioned, in of a sun- 
such a manner that the centre of the umbra of the small ^^'' 
spot then visible fell on the middle of the slit in the 
screen, which, like the corresponding one in the spectro- 
scope, was longer than the diameter of the umbra, the 
solar spectrum was observed in the field of view of the 
spectroscope, with its central portion (corresponding to the 
diameter of the umbra falling on the slit) greatly enfeebled 
in brilliancy. 

All the absorption-bands, however, visible in the spectrum 
of the photosphere, above and below, were visible in the 
spectrum of the spot; they, moreover, appeared thicker 
where they crossed the spot-spectrum. 

I was unable to detect the slightest indication of any 
bright bands, although the spectrum was sufficiently feeble, 
I think, to have rendered them unmistakcably visible had 
there been any. 

Should these observations be confirmed by observations Theab-^ 
of a larger spot free from "cloudy stratum," it will follow, 5(>*'p^ioH 
not only that the phenomena presented by a sun-spot are probably 
not due to radiation from such a source as that indicated by ''^* 
M. Faye, but that we have in this absorption-hypothesis a 
complete or partial solution of the problem which has with- 
stood so many attacks. 

The dispersive power of the spectroscope employed was 
not sufficient to enable me to determine whether the de- 
creased brilliancy of the spot-spectrum was due in any 
measure to a greater number of bands of absorption, nor 
could I prove whether the thickness of the bands in the 
spot-spectrum, as compared with their thickness in the pho- 
tosphere-spectrum, was real or apparent only.^ 

* Irradiation would cause bands of the same thickness to appear 
thinnest in the more brilliant spectrum. 



~ -I ^csi^ ^riints^ 3in<:n^ -Hfcii. S afcaS faop^ S pennitlesqr 
^;-- ~ju: n±=uii= Gt~ r.LLicofaKxvatians b cfa i g tbe Roy*^ 
ir.c;cr; - Seeing niat -pectHMK ana^F» bas alicady bevn 
applied :n -±ie jclt^ ^ini so^ imbo^ it is not too much 
V. riunk niat u? ,ic:ent:ve and iritMnWapectRBCOpic cxanf- 
aosi^n ot zae sun 3 furaoe nqr brag "* on^ koowlet^ 
b^ar-.a-^ -.a the phyv.cai oDBStitiibao of lint InnuDuy:- 
F:.r La.--Ci3ii2. if the theory of afaaorpciaa be tmC, vc aay^ 
iuppi^^e ihac in 3. d^^-z spat M3IJ% wi^it be absorbed wtiidv 
w.u.i^ isciziK: 3bx:rzz.oa. m Ihe a^a stxa£a of tbc aCmo- 
ipher± 'n^cice also the <brfaKas of a line may depend 
si-jOica bar oa the d^ tli of Uk atsort i ng atma^hcre. May " 
^■;C iU.j ic-cie of the v-ariable Boes visible in the solar 
.'•pectruin be due ta abairptioa in tbe re^on of spots *^^ 
and ina? ^^:lC the s^ ectnaacope affbtd as evidence of thi _ j 
f eiia^eace rjl the "red Samcs* «btcfa total eclipses bavg=5 
r^'.ealed tj u^ ;n th- sun's atmosplicre ; although Uic^^b^ 
e^'jiys ii'. jther mc'.fc I'is of observatkns at othcf times 
ani :: ;o, Tr.iv we r. t Leara something from this of tJi^ « 




OBSERVATORY WORK. 439 



Second Paper, 

Brief Announcements. 

•ce of an Observation of t/ie Spectrum of a Solar Promi- 
nee, by J, N. LOCKYER, Esq., in a letter to the Secre- 
ry. From the "Proceedings of the Royal Society," 

0. 105, 1868. Received Oct 20, 1868, 

October 20, 1868. 
[R, — I beg to anticipate a more detailed communication 
nforming you that, after a number of failures, which 
e the attempt seem hopeless, I have this morning 
xtly succeeded in obtaining and observing part of the 
trum of a solar prominence. 

s a result I have established the existence of three 
It lines in the following positions : — 

1. Absolutely coincident with C 
I. Nearly coincident with F, 

I. Near D. 

le third line (the one near D) is more refrangible than 
nore refrangible of the two darkest lines by eight or 
degrees of KirchhofT's scale. I cannot speak with 
mess, as this part of the spectrum requires re- mapping. 
lave evidence that the prominence was a very fine one. 
e instrument employed is the solar spectroscope, the 
; for the construction of which were supplied by the 
mment-Grant Committee. It is to be regretted that 
nstruction has been so long delayed. 

I have, &c., 

J. Norman Lockyer, 

tcretary of the Royal Society, 



SECOND 
PAPER. 



SOLAR PHYSICS. 



Tlie-je rc;uUs were announced to the Paris Academy o^E= 
Sciences b\- Mr. W'arrtn De la Rue, in the following paper 

Sur titi m/thi>dt emphyh par M. Lockyer p«nr oiurvtr en trmfr^^^^ 
i-u:;K^:rrf U sfectre dis protmiirataes iigKaltes daat Us /difuF^^ 

.v/.;.Vi iw S.-Ial. 

"J'ai eu ie pUisir de ccimnuiniquer ^ M. Delaunay deux Leitm-^^s ' 
relatives a «nc d&ouverie d'un de mes amisau snjel dcs proluWrftnccj — ^ 

roses qui *e voient pendant les Eclipses totales du Solcil. M. J 

Norman Lock^cr. en so scn'ant d'uil spectroscope consmiil Mpfb,i-^^ffl 
pu objcner les lignes brillanles d'une proluWrance superposws surr^iH 

le speclie ordinaire, qu.ind, en pafcoumnt Ic bord du Soleil, nnstni I 

mtnt se trouvail sur un tel objet. Ceiie d(5couvcrte a iii faiie 1^^3» 

/■n;K,^-r-: Lcllr,:^M. Balfgw Stewart i M. W. De jU Rm. \ 

"ii octobre 1868, I 

" Ldtkyer ^ cu un triomphe ; il a trouvtf les flammes rouj^s iv(^™ t 

son nou\e.m spectroscope. II dit, 20 ociobre ; 
"J'.ii sai^i une protuberance auJDurd'hui avec le nouveau ipccirw^EO- 

scijpe et obtenu les positiims de trois raics ; 

■■ In..-, dt S nu f) A(ii,tii dc t'&helle de Kitchhoff, plus nffrangiiiL^Clc 




OBSERVATORY WORK. 



441 






B vcxTCz ^ildonne une longueur plus graude a trots rates ; je pcnse 
: toutes les autres raies qu'il a vues viennent de cette partie tr^s- 
lantc du spectre solaire ordinaire, que Ton voit quand on observe 
region juste au delk du bord du Soleil. II me semble que cette 
4ication est d'autant plus probable que M. Rayet a observe avec 
! fente tr^s-large. Du reste, il existe dans le spectre solaire une 
ion extr^mement brillante, entre les deux raies les plus rdfrangiblcs 
^, exactement dans la position ou M. Rayet place une raie courte ; 
ndste aussi une partie tr^s-brillante entre ^ et F ou il place une 
re raie. 
^ £n r^sum^ : 

J'ai d^termin^ trois raies : 

Rayet donne trois raies plus longues que les autres ; 
^ 3* Tennant est sdr de trois raies ; 
'4* Herschel est sdr de trois raies. 

'II me semble que les lettrcs de Herschel et de Tennant et aussi le 
gramme de Rayet font voir que leur nomenclature repose essentielle- 
nt sur une estimation plus ou moins rigoureuse, et non sur des 
Sures ; aucun de ces Messieurs ne parait avoir pensd h mcttre dans 
^hamp de son telescope une dchelle faiblement illumin<5e. 
*Ainsi mes trois raies peuvent, apr^s tout, repr^senter une plus 
•ude portion du travail accompli que je ne I'avais d'abord imaging. 

* Avec une fente ^troite, les raies ont ^t^ vues jusqu'k une petite 
lance sur la surface meme du Soleil. C est de beaucoup la raie 
^s brillante, et Mme. Lockyer a pu Tapercevoir sans difficult^. 

* Les raies se prolongeaient k des hauteurs diflfi^rentes au delk des 
"ds du Soleil ; la rouge dtait la plus courte. J'ai meme pu me rendre 
npte de la forme de la protuberance qui a dCi ^tre celle qu'indique 
te figure : — 




Fig. 141. — My first prominence. 



■Quand la fente a M ajust<5e de mani^re h tombcr sur /7, la raie 
Uante s'est trouvde enti^rement s<5par(5c du spectre solaire. 
"Voici un autre fait : quoiquc C et F soicnt considdrdes toutes 
■X comme dtant les raies de Thydrog^nc, clles n'avaient pas ce- 
idant des longueurs dgales : la rouge s'approchait davantage du 

The reading of this letter was followed by a communica- 
^ from M. Janssen, received the same day. I cannot 



SECOND 
PAPER. 

Lines 
observed. 



SOLAR PHYSICS. 



SECoNt) refrain from printing this, and M. Faye's remarks on botli 
"'''^''' the conimunicatioiis, in this place: — 

Im/iciitipti lie quelques-uns del risul/ats obtenut i GuMtoor^ fi<ndalji 
r Idipsc du mois d' <ioi7t dernier, el i la suite de cette ieiipu. Utttf 
d( .1/. Jaiusiii A M. le Secr&aire Perpeluei. 

"Cocanada, 19 septembre 186S. 
^f, "J'^rrivi; en ce moment de GuDtoor, ma station d'obscrvaiion de 

Juiiisin's rjclipse, ct je profitc i la hitc du depart do courrier pour lionDet 
/irt.T. ?i I'AcadL'mie dci nouvelles de la mission qu'elle m'a fait I'tionDm 
dc mc conticr. 

'■ Li' Uinjis me manque pour envoyer une relation di^tailliic ; i'auni 
I'luinneur de ia f.iiri: par le procliain courricr, Aujourd'hui jc runmc 
rji SL'ulcmcnt les pniicipaux r^uhals obtenus, 

" La station dc Guntoor, a i\t sans doule la plus favorisife ; le dd 
a i5ti.' boau, sitrtoiit pendant la total it d, et nies pulssantes luQctiMde 
prts dc 3 metres de foyer m'ont pemiis de suivrc I'dtudc analytique de 
tous Ics ph(!nomi;nes dc i'^clipse. 

'■ Immi'diatcmcnt nprJs la totalit<^, deux magnifiques protuberances 
ont ^pp.iru ; I'linL- dclles, de plus de 3 minutes de bautetir, briUaiE 
dune splcndcur qtiil est difficile d'imaginer. L'analyse de sa lumifre- 
m'a immodintement montrtf qu'elle (Sail forni^ par une iroinefoc 
^M^cu^o incandescente, principalement cgnipostfe dc gas 




-•.> 



OBSERVATORY WORK, 443 



'' Je suis heureux d'offrir ces rdsultats k TAcad^mie et au Bureau des second 
Lon^tudes, pour r^pondre i la confiance qui m'a ^t^ t^moign^ et k paper. 
lliomieur qu on m*a fait en me confiant cette importante mission." 

M. Faye*s remarks were as follows : — 

"Je demande la permission d'ajouter quelques mots k Vexpos^ si M.Fayis 
lucide de M. le President, pour expliqucr la singuli^re coincidence des remarks, 
deux communications qui viennent d'etre faites k I'Acad^mie. 

" II est certain que Tid^ premiere de la m^thode par laquelle M. 
Janssen d'abord, puis M. Norman Lockyer, sont parvenus, Tun aux 
Indes le 19 aodt, Tautre en Angleterre le 20 octobre, k saisir par 
FansQyse spectrale et k mesurer des phdnom^nes invisibles jusqu'ici, a 
A^ ixxxagin^e et propos^e en premier lieu par M. Lockyer, mais elle 
n'avsut conduit k aucun rdsultat. Tout ce qu*on pouvait conclurc des 
premieres tentatives faites dans cette voie nouvelle dtait une negation, 
Qoant k la nature gazeuse des protuberances. Or cette conclusion, 
dhr£€Pri peu admissible, a dii jeter quelque ddfaveur sur la mdthode 
dlc-ix^^me. Voilk ce qui m'explique le peu d'attention que les obser- 
vate^rirs de F^clipse ont donnd k cette mdthode, publi^e depuis deux 
ans dans les Proceedings de la Societe Roy ale de Londres. Les astro- 
nomes anglais eux-memes ont ndgligd d'en faire ^application dans 
Icnr ^3cp^ition des Indes k la suite de Tdclipse du 18 aoiit. 
,"1— *insucc^s des tentatives premieres de M. Norman Lockyer (il est 
*is^ de s'en rendre compte aujourd'hui) me parait tenir k ce que ce 
s^VM^t, dans Timpossibilit^ oil il ^tait alors de prdvoir de quelles raies 
liimii:x«uses se composerait le spectre des protuberances supposdes 
gazeiases, ne savait sur quelles particularitds ddlicates du spectre si 
^^^'^I^lique des regions circumsolaires il devait porter son attention. 
^1^ «st si vrai, que c'est seulement quand il a su, par les observateurs 
™*^^s et anglais de I'dclipse, la nature ddtaillde du spectre des pro- 
^D^«'«fcnces, qu^il a rdussi k trouver en Angleterre les traces de ce 
spcc^x^ dans celui des rdgions voisines du bord du Soleil.i 

^^L.*id^ de la mdthode a did exposde pour la premiere fois dans un 

Mecfcx^ij-e conmiuniqud k la Socidtd Royale, le 1 1 octobre 1866, sous le 

??K.» " Spectroscopic Observations of the Sun, by Norman Lockyer." 

^-opj^ pnncipal de ce Mdmoire dtait Tdtude du spectre des taches, 

"^^ Tauteur finit par ces mots : " and may not the spectroscope 

*®?'*'d us evidence of the existence of the red flames which total 

^^I^Ses have revealed to us in the sun's atmosphere, although they 

*^*^1>« all other methods of observation at other times ? and if so, 

®*y yre not learn something from this of the recent outburst of the 

*^ in Corona?" L'auteur ne s'est pas contentd d'indiquer cette 

™^J*^cde : il Fa appliqud avec persdvdrance pendant deux ans k la 

'^^^Tche des " red flames." Malhcureusement il aura sans doute 

P*^^U de vue Fanalogie ou'il avait si bien signald lui-m6me, entre le 

J?^^tre de I'dtoile merveilleuse de la Couronne et le spectre probable 

^^ I'^gions circumsolaires occupies par des protuberances, analogie 

?^^riue si frappante depuis la derni^re dclipse. Aussi avait-il dtd 

torc^ de conclure, par Tinsucc^s de ses premieres tentatives, k la non- 

^^itd des protuberances. M. N. Lockyer a meme reproduit tout 

'^^xximent cette conclusion en juillet dernier, dans un article que 



ff m Solar 1 
SosmftS Loccra. FJLA^ in a k 
farDc: Sa 




at an i 
I sides. I may add 
iMit oat that we may 
Ike tEsapcratsre of l' 



J. XORWAX^ 



imiaiC 4km£ SMS f IKULJ rappoRs 

' u ^J|*a lOfQ : * la Ife fast pboe a djii 




OBSERVATORY WORK. 445 



Complete Account. 

Sfecr^^oscopic Observations of t/ie Sun, No. II. — By J. N. secokd 
Lcxkyer. Communicated by Dr. Sharpev, Sec R.S. ^^^'^' 
R.^ceived November 19, read November 19 and 26, 1868.^ 

Preliminary Remarks. 

Iki. my first paper under the above title, kindly com- 
in*u^icated by Dr. Sharpey to the Royal Society in 1866,* 
was contained an account of the determination of the 
nat^^ire of sun-spots, by means of the spectroscope. The 
pap^r concluded as follows : — 

Way not the spectroscope afford us evidence of the 
cxisticnce of the ' red-flames ' which total eclipses have 
'"cv^^led to us in the sun*s atmosphere, although they 
^sc^pe all other methods of observation at other times } 
^^ if so, may we not learn something from this of the 
''cc^nt outburst of the star in Corona } " 

Before the paper was written I had diligently swept 
roai^d the solar disc in search of evidence of the red flames, 
but ivithout result. This want of success I attributed to 
.the excessive brilliancy of the spectrum of the circumsolar 
"^lons in the field of view of the instrument employed. 
I fovind in fact (although I did not discontinue my efforts) 
^^t both for these observations and for those on sun-spots 
nioj-^ dispersion was necessary ; in one case to weaken the 
^^^^Dspheric light, in the other to widen the spectrum. 

I therefore represented my requirements to the Govern- Aid sought 
™^^t-Grant Committee, and was at once supplied with •^'^^'^ ^^\ 
"**^<3s to procure a spectroscope of the requisite dispersive Grant, 
P^^*er. 

^*he construction of this instrument was commenced in 
^^ beginning of 1867 by Mr. Cooke, on a plan which 
*^^ been arranged between us, but unfortunately it was 

* Phil. Trans. 1869, p. 425. 
* Proceedings of the Royal Socit'ty, vol. xv. p. 256. 



J-''^— iiaii:^. inti It iii; at fte end <f iIk year, he beggd 
■:™» r ^ne 3s lanswu^ die irder rancttfcJ . C'sder tfaese d 
^^^^ taxless, ir :iie b.=rnnSie ** ** p wie»C year I j 

' v.^M~~ :''jr±er ieiayed. ^ait^ «■ »'"Tif^ of as Soess 

T^^-^iiacei Tiy ^boeiKe finn ftg^iiA At lut iIk 
.ti^trimcnr. vtiich ;cflxfi CK>C oedtt on Hz. BnmJBe's 
ski!. ii-Tirai rm the Ah sf Ocfetifief; i868b oat qnk 

^^mptitti. bur m i eooAiatt iAhA rrnHM me to o 

I -^anrijca di<^« ^cB^ Sot Id J cc BMt for my apputiA 
;iia>:=i:cL mii ^'irDudly a acdor Oat t&e coinddeaoe t 
dn:>: f my rsslts wiEb Anae tibfjiiin] by the obsemTS 
Gi ^''.e rscesc eclipse magr aot be aisattcrpreted.* 

I Gt^^^n tlv vonc with Ac near insliuioeid by c 
T>_J3^-i!7 -.;ir:ii after the pceaRBences. I found that j 
: —--r- ^r if^^ht was bow so greatly reduced I 
._" '.'.-.: r.ii were Einitiy seen oo a dimly i 




OBSERVATORY WORK. 447 



adjustment, were as unsuccessful as those made with the second 
smaller one ; and it was not till the 20th of October that, ^^^^^ 
after sweeping for about an hour round the limb and jt^ 
arriving at the vertex of the image, near the south ^^'T^ 
pole of the sun, I saw a bright line flash into the field. Unafirii 
My eye was so fatigued at the time that I at first doubted '^** ?• ^' 
its evidence, although, unconsciously, I exclaimed "At oooAer 
last!" The line, however, remained — ^an exquisitely '^^• 
coloured line absolutely coincident with the line C of the 
solar spectrum, and, as I saw it, a prolongation of that line. 
Leaving the telescope to be driven by the clock, I quitted 
the observatory to fetch my wife to endorse my obser- 
vation. 



Detail of the Observations, 

October 20. — Having settled that the new line was Search for 
absolutely coincident with C, I commenced to search *'^^^*^' 
for more lines. This I found very difficult, as the instru- 
ment requires several movements and adjustments for the 
various parts of the spectrum, and the rate of the driving- 
clock was not properly adjusted for the sun's motion ; the 
prominence was therefore lost at times ; moreover the 
observations were impeded by clouds. 

I commenced the search for lines from C to A. B was 
first brought into the field with the newly discovered line 
at C. There were no new lines visible. I then made an 
excursion to A with no result, and returned to C to assure 
myself that the prominence was still on the slit. 

I then worked from the line at C towards D. A little 
beyond D, the lines of which are widely separated in my 
instrument, I detected another single and less vivid line, by Results. 
estimation 8° or 9** of Kirchhoflf's scale more refrangible 
than the more refrangible of the strongest D lines. I could 
detect no line corresponding to it in the solar spectrum, but 
the definition was not good. 

b was next tried, the excursion now being made from the 



SOLAR PHYSICS. 



AfPctr- 
vf Ihi I" 



new line near D. There was no line at b, though thenew D 

. line was still visible when I returned to it 

In the same manner, after many interruptions from 
clouds, I tried F ; here I found another line. As at first 

f caught it was very long ; and by moving the telescope vey 
slightly backwards and forwards in right ascension, in one I 
direction the line shortened and brightened, and was visible: i 
on the solar spectrum for some distance, in another direc- 
tion it became disconnected with the spectrum altogether. 
I was hence able to determine roughly the shape and 
dimensions of the prominence. 

It was extremely difficult to fix the exact position of the | 
line at F, although I bad had no difficulty or even cause for , 
hesitation about the others. It seemed at times to Ii«i 
athwart the K line in the faint spectrum, although at first it | 
had appeared more refrangible, especially when it nus 
visible on the solar spectrum itself 

October 22, — Two days afterwards, I had another 
opportunity of observing the prominence spcctnjm, and i^i 




OBSERVATORY WORK. 449 

pse the dark line in the spectrum, and to replace it by a second 
dly bright band (Fig. 89), The behaviour of the F ^^^^^' 

was still a puzzle to me. In the spectrum of the Remarks 
t proceeding from the exact limb of the sun the /^{^^ 
ht line was seen more refrangible than F, (Fig. 100,) thtY lime, 

in the spectrum of the prominence at some distance 
i^ the sun the black line F was eclipsed. This 
crimen t, which I repeated several times, seemed in 
casure to explain what I had before observed ; and 
er this date I entered in my note-book, " It appears 
away from the sun's surface the substance gives 

less refrangible light than it does when apparently 
le surface." 

ovember 5. — The next observations were made oft 
date under superb atmospheric conditions, and after 
mportant alteration had been made in the instru- 
t, enabling me to make the several adjustments with 
itmost nicety. 
fter the adjustments to the sun's lirt^b had- been made, 

once saw what I imagined to be the indication of 
lall prominence, and swept for a development of it, 
cing that the portion observed might be one of the 
» or lower levels which- generally separate the higher 
:s. Having swept for some distance on both* sides the 
m on which the telescope was clamped in the first 
ince, and finding everywhere the same uniformity 
eight, it at once struck me that I was in presence 
>inething new, and that possibly what I was seeing 
It indicate a solar envelope. I rapidly, therefore, 
I several other parts of the limb to test the idea. 
as soon established. /// every solar latitude both the C TheC and 

F bright lines were seen extending above the solar j^/J^^ 
rum. The spectrum near D was so bright that I was tvery solar 
pdled to refrain from examining it, but I caught ^*^ ' 
line near- D once. The thickness of this envelope I 
d to be sensibly uniform, except in the regions where 
IS heaped up with prominences. 

G G 



.VUZW* fHYSICS. 



The spectrum of the envelope cleared up all the 
difficulties connected with the F line. Perfect definition 
. and adjustment now enabled me to see that the bise 
of the bright line widens out as the solar spectrum ii 
approadied, and that whereas the Hne, away from the 
sun, corresponds, in the case of an ordinary prominena, 
in refrangibility and thickness to the Fraunhofer line F, 
close to the sun it widens out, so as to overlap the F line on 
both its sides to an extent about equal to its thickness, so 
that it is tliree times broader, or perhaps more, on and 
close to the limb. 

In the spectrum of a prominence in which violent actioo 
was goin^ on the line tliickened out in the same manneril 
some distance above the limb (Fig. 91). There was 
no thickening observ-ed in the C line at the base, or in 
the case of the phenomenon j'u-st referred to, 

I obtained on this day the outlines and dimensions of 
two iiromincnces. 

Xovi'mbcr 6. — The observations made on the preceding 




OBSERVATORY WORK. 451 



On tJie Spectrum of the Promincfices. 

The existence of three lines in the spectrum of the second 
prominences and their approximate positions were deter- _Z^fl^_ 
mined and communicated to the Royal Society on the 
20th of October. 

The coincidence of one of the lines with the solar line C The coinci- 
was at once determined. ^^^ ^ 

The coincidence of another line with F at a certain mosphenc 
distance from the sun's surface was finally determined on '^^ ^'^ 
the 5th of November, when the fact of the widening out of determintd 
the lines towards the sun was discovered. 

The exact position of the line near D is shown in Fig. The exact 
87, in which it is laid down from the mean of three position of 
careful micrometrical measurements made under far from ^ined, ' 
good atmospheric conditions on the isth of November. 
In KirchhofTs map the new line falls in a region where 
no line was measured by him. I may also add that, 
by the kindness of Mr. Gassiot, I have been enabled to 
inspect the very elaborate maps of the spectrum con- 
structed at Kew Observatory. The measures above given 
make the new line fall between two lines of almost in- 
conceivable faintness ; in Mr. Gassiot's map, indeed, there 
are none but such lines for some distance on either side of 
the region in which the new one falls. 

On the 8th of November the existence of another line The exist- 
was definitely established; its position in the spectrum 'J/^-d^ 
is slightly less refrangible than Fraunhofer's C. Cestab- 

Unlike the other lines, which are seen in all prominences, " 
this line is only visible at times, being rendered so appa- It is very 
rently by the presence of certain conditions which are not 
permanent. Intense action going on in a prominence will 
sometimes render this line visible : I am not, however, pre- 
pared to say that this is always the case. The line when 
visible is much more variable than the others : at times it 
is the mere ghost of a line, at others it rivals the C line 
in brilliancy. 

G G 2 



soutK fnysiCs. 

Oi ihe three lines C is generall)' more brilliant than F ; 
but 1 should 3i:d that it is difficult to determine the rc- 
Iativ£ brilliancy of the lines because they are never seen 
logethsr in the field of view of my instrument. The 
relative bniiiancy oi the line, near D, i am not sure aboot, 
because its situation in the brightest portion of the spec- 
trum not onlj- renders comparison difficult, but rcndcn 
^ny cvtndiision which may be formed little worthy of 
o.'nndercc. My ubservations so far (November i6th) in- 
.iuce nie to ascribe gteat variability, not only to the 
absolute, but to the relative, brightness of the lines. One 
instance is of the utmost importance. On the 5th of 
November, in sweeping round the sun with the F line in 
the field of view. I came across a prominence in wfaJdi 
acliiin of the intensest kind was evidently going on ; the 
light and colour of the F line were most vivid, the lumin- 
osity of the line was greater than that of any part of the 
s.'lar spectrum then \-isible in the field (Fig. 91). The ac- 
tiini was Hut yiineral, but limited to certain points, a:; if 




OBSERVATORY WORK, 



453 



of observation (the 22nd), as the sun's rotation had carried 
it on to the disc. 

On the 5th of November 1 obtained the outline of two 
brilliant prominences, one near the southern, the other 
near the northern limb of the sun. The extreme (measured) 
apparent height of one was 35,000 miles, of the other 
somewhat less ; the former I estimated to extend along the 
sun for about 200,000 miles. The shape of the southern 
prominence had changed considerably by the next day, the 
bright peak being quite gone ; at the same time the length 
of the main portion had extended as if the peak had been 
absorbed into it. 

The prominences therefore change from day to day ; at 
present I have not measured any more rapid change, but 
these observations are of so delicate a nature that it is 
easy to imagine rapid changes to be going on in any 
prominence of decided outline ; for an error in the adjust- 
ment of the instrument with regard to the meridian or 
latitude, the least variation in the rate of the driving-clock, 
or any oscillation of the telescope-tube or the spectroscope, 
brings the slit on another part of the outline of the 
prominence, and under these circumstances the length of 
the line is perpetually varying. 

It must be borne in mind that the dimensions of the 
prominences cannot be determined absolutely, as we do not 
know whether they are actually on the sun's limb at the 
time of measurement. Measurement can only fix a 
minimum. 

Ojt the Continuous Solar Envelopes, 

The continuity of this envelope, which I propose to 
name the chromosphere, a name suggested by Dr. Sharpey, 
was spectroscopically established on the 5th of November, 
and an account of the observations was transmitted to the 
Royal Society on the same day. 

By careful estimations made on the 6th of November 
(which are estimations only, for I had not yet mounted 



SECOND 
PAPER. 

The di- 
mensions 
of some of 
the promi- 
nences 
measured. 



The pro* 
mtnences 

change 
from day 

to day. 



The con- 
tinuity of 
the enve- 
lope is es- 
tablished ; 
it is named 
the Chro- 
mosphere, 



SOLAR PHVStCS. 



a wire-micrometer on tfic spectroscope- telescope), its 
general thickness «as determined to be about 5000 miles; 
the level of its upper surface was not absolutely uniform in 
all latitudes, but it was very neatly so, I could detert 
no difference in the general level as between the equa- 
torial and sobr regions of the sun. 

It would appear that the light by which its existence 
is revealed proceeds from the same substance or substance? 
of which the prominences are composed ; and I hold the 
prominences to be merely the heaping together of the ncMf 
envelope in some localities, 

Under proper instrumental conditions the spectnim of 
this envelope can always be seen whenever the sun is 
shining. The spectrum consists of a line coincident with 
Fraunhofcr's C, another more refrangible than D. itid 
another coincident in the main with F. I say coincident in 
the main, because when the spectrum of the envelope 
is viewed so that it appears to rest on the solar spectrum. 
the line at K takes the form of an arrow-head IFif. 




OBSERVATORY WORK. 455 



SECTOR D 

On certain Bright Regions in the Solar Spectrum, paper. 



From the commencement of my observations with 
the new instrument my attention has been drawn to certain 
bright r^ions in the ordinary spectrum ; but it was not till 
the 8th of November, 1868, that I succeeded in observing 
a definite bright band extending for a certain distance on 
the sun near the limb. 

I should state that I have observed this behaviour in the Tke F amd 
F band on either side of Fraunhofer's dark line F, and ^Z'^^*^ 
in the C line, when the prominence, as I have imagined, has tke limb 
extended from the limb over the earth's side of the sun. *''*^- 

The position of the bright band observed on the 8th of 
November is near C, but slightly less refrangible, not far 
from the place in the sdale occupied by the last discovered 
red line, the position of which as yet has not been 
micrometrically determined. 

Other regions to which my attention has been particularly 
drawn from the first, although up to the present time I 
have obtained no results, lie, one between the b lines, 
another between b and F, another less refrangible than B, 
one near D, and another near G. 

It is quite possible that these bright regions, the light Bright 
of which is variable, may be due to facula? ; this conclusion regions 

1 i t t r 1 i-i- . • 1 . tnay be due 

IS Strengthened by the fact that diligent sweeping within tofacuUe, 
the limb has not revealed the bright lines of the chromo- 
sphere spectrum. If this be so, the faculte are not the 
prominences, although they may be possibly connected 
with them. 

On the nature of the Chromosphere a fid Prominences. 

It has already been concluded by M. Jansscn, from the 
coincidence of two of the bright lines with C and F, that 
the prominences are composed of hydrogen. 

So far as our present knowledge goes, however, this does 
not dispose of the other two bright lines, the positions of 



SOLAR pjjys/cs. 

which have been determined by myself: I allude to the 

. lines near D and near C. 

At the present moment I am engaged on a scries of 
experiments on gaseous spectra, which 1 hope wiU afford 
additional information on these points: in the interim. on 
the assumption that the chromosphere and prominences 
are wholly, or in part, composed of hydrogen, several con- 
siderations which appear to me of great importance may be 
toucht^d upon. 

These considerations are based upon the experiments of 
MM. Pliicker and Hittorf on the one hand, and of 
Professor Frankland on the other.* In MM. Pliicker ami 
Hittorf's paper entitled " On the Spectra of Ignited Gases 
and Vapours, with especial regard to the different Spectra 
of the same elementary Gaseous Substance.'" these investi- 
gators point out the effect of temperature on the different 
spectra, the temperature of the discharge of RuhmkoriTs 
induction-coil being increased by increasiniJ the power of 
the inducing current, or, preferably, by diminishing the 




OBSERVATORY IWORK, 



457 



Ho, H/8, and H7 by MM. Pliicker and Hittorf. The places 
of these lines in the solar spectrum are at C, at F, and one 
at some distance from G towards F. 

I quote the following results of increase of temperature 
from the memoir under notice : — 

" Hydrogen shows in the most striking way the ex- 
pansion of its spectral lines, and their gradual transforma- 
tion into a continuous spectrum. When the direct discharge 
of RuhmkorfTs large induction-coil is sent even through 
the old spectrum-tubes enclosing hydrogen, the formerly 
obtained spectrum is essentially altered. By increasing 
the power of the coil, the violet line H7 first expands ; 
while it cjntinues to expand, the expansion of the bluish- 
green hne H^ becomes visible. Let the aperture of the 
slit be regulated so that the double sodium-line will separate 
Into two single lines nearly touching one another. Then, 
the angular breadth of H^ becoming two or three minutes, 
the breadth of H7 is about double. The expansion takes 
place as well towards the less as towards the more refracted 
pout of the spectrum. Ha remains almost unchanged 
after H7 has passed into an undetermined large violet band, 
and H/d extended its decreasing light on its two sides. 
On employing the Leyden jar, and giving to the gas in 
our new tubes a tension of about 60 millims., the spectrum 
is already transformed into a continuous one, with a red 
line at one of its extremities. At a tension of 360 millims. 
the continuous spectrum is highly increased in intensity, 
while the red line. Ha, expanded into a band, scarcely 
rises from it. If the electric spark passes through hydrogen 
at the ordinary tension, the ignited gas on its way always 
gives the spectrum of the three expanded lines. 

" Even in the old spectral tubes inclosing highly rarefied 
hydr<^en, the ground, from which the three characteristic 
lines rise, did not appear always of the same darkness ; 
in some instances new bright lines appeared, especially in the 
neighbourhood of the sodium line. In resuming the subject, 
we pointed out the existence of a new hydrogen-spectrum, 



SECOND 
PAPER. 



PUuka^s 

and 
Hittarfs 



New bright 
lines seen 

in the Spec- 

trum of 

hydrogeti. 



SOLAK PtfVSICS. 



correspond in^r to a lower temperature, but having nc 
. resemblance at all to the spectra of the first order of 
nitrogen, sulphur, &c. In this spectrum, of a peculiir 
character, if fully developed, we observe a great number of 
well-defined brisjlit lines, almost too numerous to count and 
represent by an engraving, but brilliant enough to be 
examined with a magnifj-ing-power of 72. after the light 
has passed through four prisms. 

"On sending the direct discharge of Ruhnikorff's oril 
through a tube of glass from one-fourth to onc-eighlh of 
an inch in diameter, provided with electrodes of ptatioum 
or of aluminium, inclosing hydrogen at a tension of S 
to 10 millims,, a luminous thread of light of a bluish-white 
colour was seen passing along the axis of the tube, without 
touching the glass. When analysed by the prism, it gave 
a faint spectrum of the above-mentioned numerous brigbl 
lines, especially within the red and the yellow. Among 
these lines neither Ha nor H7 were seen ; H$ only ap- 
penrcd, but less bright than many other lines. By inter- 




OBSERVATORY WORK, 459 

With hydrogen gas in Geissler's tubes, then, the following sicond 
facts are established :— ^^^^^' 

I. With a certain degree of rarefaction and temperature, 
we obtain three characteristic lines, Ha, H/J, H7. 

II. By increasing the temperature, we expand H7 first Effects of 
towards both ends of the spectrum, then H)8, Ha remain- ^^^^p^ 
ing almost unchanged after H7 has passed into an undeter- ratureon 
mined large violet band. ^^ge^^st^. 

III. By increasing the tenuity, Ha disappears first, Wfi tmm, 
remaining well defined, and moreover the colour of the 
ignited gas changes to the eye. 

IV. Under certain conditions, which are not stated in the 
memoir, new lines appear in the spectrum, especially in the 
neighbourhood of the sodium line. 

Assuming that hydrogen gas is present in the chromo- 
sphere and prominences, we have the following facts to 
place side by side with those just stated : — 

I. In place of the three lines we have but Ha and 

II. up is in process of expansion, the expansion in- 
creasing as the sun is approached, and H7 is so far ex- 
panded that it no longer exists as a line; most careful 
observations repeated several times have failed to detect it. 
Were it a broad band having the same total amount of 
light, it would be invisible in the spectroscope ; it has 
probably therefore reached this stage. 

III. The prominences have been observed of various 
colours (this fact is not here stated merely with reference to 
the observation recorded in HI. afite). 

IV. There is a line in the yelloiv, most probably pro- The length 
ceeding from the substance which gives off the light at J^'^V ^]» 
C and F, as the length of this line, as far as the later same as 
observations with the more correctly adjusted instrument '^''i^-^S 

1 1 /• 1 • X- 1 f-. and of Y, 

go, is the same as that of those m C and F. 

I am aware that the conditions as to density cannot 
for one moment be held to be the same in the two cases ; 
but as at present (so far as I know) we have no similar 



SOLittt PHYSICS. 



experiments ranging over greatly x-arying densities, I 
have thought it desirable to bring these striking facts for- 
ward at once. We are justified in thinking that the 
density of the chromosphere, atwa^ys assuming that it is 
composed wholly or in part of hydrogen, cannot be very 
great : if it were, the spectrum would most probably be 
continuous; for Professor Frankland has shown, in ^c 
" lecture before alluded to, that hydrogen burning in oxygen 
under a pressure of ten atmospheres gives out a speclrum. 
bright, and perfectly contiDuous from red to violet. 

It is possible that experiments in which both density 
and temperature are varied may enable us to match accu- 
rately the spectrum of the chromosphere, and thereby 
determine both the temperature and pressure at the surface 
of the sun. 

The bright lines which have been observed in several 
stars, especially in the remarkable one in Corona, the out- 
burst of which was spectroscopically watched by Mr. 
Huc-iiins in iS^GG, show us tliat under certain condit'ons 




OBSERVATORY WORK. 461 



Spots, and that nearly the whole limb of the sun was second 
covered with prominences. My observations since 1866 ^^^^^ 
have been carried on at the minimum sun-spot period, 
and the prominences observed during the eclipse this year 
were few in number, and covered but a small part of the 
sun's limb. 

Additional Note oti the Chromosphere} 

Since my last communication to the Royal Society I 
have received, through the kindness of Admiral Manners, 
the following extract of a note from Father Secchi, in 
which, although the existence of the new continuous enve- 
lope is not announced, important corroboration of its 
existence is contained. Father Secchi says ; — * 

•' Si Ton met la fente parallel k la tangente du bord .... ^ , 
en arrivant plus pr^s du bord la ligne devient continue. SecchCs 
Cette observation prouve que la couche gazeuse rose est ^*^'' 
continue, mais tr^s-irr^guli^re dans son contour, comme 
Tont montr6 les Eclipses." 

^ RomCy November 15M, 1868. 

•• I have been able to verify the observations of Mr. 
Lockyer on the sun, but I find that, even where the lines 
do not become brilliant, their blackness vanishes by a 
partial inversion. I have foimd also some luminous lines 
which become exceedingly brilliant near the edge of the 
sun. One is near the ray D, and the other in contact 
almost with the line B on the side of C. 

" / find that all around the full limb of the sun the inver- 
sion takes place. If the slit is perpendicular to the edge 

the inversion is a very short part, so 

/ Fin. T4». 

of lo"" or is'", but if the slit is parallel to the edge then 

1 Received November 26, 1868. 

' Comptes Rendus, vol. Ixvii., p. 10 18. Letter of the 13th November, 
printed 25th November, 1868. 



SOluiR PHYSICS. 

the inversion is complete. It is a Very beaiitiful fut 
It is perhaps one that will modify our ideas on the origin 
of these lines," 

I have italicized the most important part of the letter. 

Since the 20th of the present month, in consequence of 
a conversation on that day with Mr. De la Rue, I have 
gone over my observations of tlie sun's limb with great 
care, and have also re-examined Mr, De la Rue's photo- 
graphs with a view to ascertain the evidence which liiq 
give of the continuity of the envelope The result strongly 
confirms my former views. It is true that the photographs 
do not show a continuous chromosphere of anything like 
uniform thickness ; but that arises from the fact that the 
only part of the sun's hmb where the envelope was visible 
at all during totality happened to be covered by irrcgulit 
prominences, which were probably very abundant at the 
time. In fact, owing to the relative sizes of the sun xni 
moon during the eclipse of 1S60, and the direction of Ihe 
moon's motion the top and bottom of the sun's limb, is 




OBSERVATORY WORK. 463 

be found to bear independent testimony to the accuracy second 
of the conclusion which I have arrived at, by an entirely ^^^^^ 
different line of research. 

Historical Notice of the growth of our knowledge of the 

Chromosphere} 

When I was first enabled, by means of the spectroscope, 
to determine that the prominences are merely heapings 
up of an envelope which gave everywhere the spectrum 
of hydrogen (at pressures which Dr. Frankland and myself 
have since approximately determined) and is continuous 
round the sun, or at all events continuous in the same 
sense that the photosphere is continuous, I was not aware 
that I had been anticipated in any part of the discovery. 

I have since found, however, that the continuity of the TheconH- 
envelope, apart from its nature and place in the solar ^l^I^L^^^ 
economy, has been suspected for many years, although longsus- 
it had never been demonstrated, as it easily might have P^^* 
been, by eclipse observations at properly chosen stations ; 
and although it has been very variously interpreted. 

I think it desirable, now that the spectroscope has deter- 
mined the existence and nature of such an envelope, that, 
in justice to myself and to those who have gone over 
the same ground before mc, a brief historical notice of 
the progress of our knowledge on this point should be 
given. 

It is easy now to divide the phenomena observed in the 
many eclipses between 1706 and 1842 into two classes: 

I. Observations of the prominences properly so called ; 

II. Observations of the chromosphere; 

amd all the observations of both these classes, accumulated 
during the eclipses which happened up to and including 
the year 1842, have been discussed by two eminent astro- 
nomical authorities, — I refer to Arago and Professor Grant, 
to both of whom, long even before the eclipse of 1851, it 

' Received April 9, 1869. 



soi^x PHVsrcs. 



was perfectly obvious that the prominences were solar and 
not lunar phenomena. 

Arago' considered the prominences to be merely cloudi 
floating in the sun's atmosphere— an atmosphere rendered 
evident to us by the corona, and to these clouds he ascribed 
the spots without a nucleus; to the corona also he attri- 
buted the darkening of the limb. He says: — 

" II faut admettre une enveloppe extcrieure qui diminuc 

■ (eticnt) moins la lumitre qui vient du centre que In 

rayons qui viennent sur le long trajet du bord a I'ofil 

Cctte enveloppe exterieure forme la couronne blanchitre 

dans ics eclipses totales d» soleil." 

It is not easy to reconcile ali Arago's statements as W 
the nature of this atmosphere or envelope; but I ^all 
return shortly to a later enunciation of them by himsdf, 
merely remarking here that there is nothing said about tiie 
'■ clouds " forming a continuous envelope round the sun. 

Professor Grant, ^ who went over the same ground U 
Arago, and had Arago's results before him, was led lo 
r bfforu the eclipse of 1851 that " The observalioi 




OBSERVATORY WORK, 



465 



le probability, obviates the necessity of intro- 
:o the theory of the physical constitution of 
le idea of a third envelope independent of the 



SECOND 
PAPER. 



n 



Swan's 

• 

tnffftotrs. 



:t come to the eclipse of 185 1, which was 

by Professor Swan, among others, who con- 

• the Royal Society of Edinburgh three valuable 

>n the eclipse, with special reference to the red 

es. Assuming that they existed in the solar 

e, and agreeing with the prescient remarks 

Herschel, that they must be "cloudy masses 

>t excessive tenuity," he remarks : ^ — 

Lisly the simplest view that can be taken of this 

on, is to regard the red fringe and the red 

es as of the same nature ; and all the observa- 

:hen confirm the idea that the matter composing 

:ts is distributed all raimd the sun. It therefore The pro- 

bable that when we are furnislied with observa- wf«^/ 

. , - - , , . - matter u 

tangential phase of the eclipse from stations distHimteit 
th side of the moon's shadow, it will be found f*[tjround 

the suft, 

rra appeared towards the sun s north point, of 
I detached prominences seen in that region, 
^ers situated near the middle of the moon's 

ere only the highest peaks Sinqe, then, 

1 shown to be highly probable that the matter 
; the red prominences is distributed with little 
•n all round the sun : we may conceive the 
;trata of the solar atmosphere to be surmounted 
elope of clouds of which the higher portions are 
yond the moon's limb, it the central phase 
eclipse, and which then constitute the red 
es. If [he continues, throwiwg out the sugges- 
Dusly made by Grant] it be thought that the 
J of two envelopes of cloud, one above and 
low the luminous strata of the sun's atmosphere, 

tions of the Royal Society of Edinbur^k^ vol xx., part iii., 
,464. 

H H 



SOLAR PHYSICS. 

iiitroduces too great complication, wc may avoid the objec- 
tion by supposing that the envelope which occasions 
tlic ptiitimbras around the spots penetrates the lioni- 
nous stratum, and exists, although in greatly difTcrea 
degrees of density, both above and below it. 

■' If, then, we cuiiceive that a stratum of cloudy nutiu 
surrounds the sun, of which the red prominences are tJic 
higher portions, the serrated appearance of the long range 
of prominences, seen by Mr. Dawes and Mr. Hind, suffi- 
ciently indicates that its general surface is exceeding!)- 
uneven, presenting the appearance of being covered with 
numerous eminences or Hdges. But these irregularities 
are small when compared with the large hook-^peij 
prominence, and its companion the detached cloud, which 
were SL-en by most of the observers of the eclipse. , . . . 
Now. as the spots have been supposed to arise from v^ 
\\ard currents causing apertures in the sun's luminotB 
atmosphere, I conceive the higher red prominences, or tbose 
which remain visible at the middle of the total phase of a 




OBSERVATORY IVORK. 4^7 



fo that the corona is also a solar appendage. We second 



PAPER. 



four envelopes : — 

dark cloud below the photosphere. (The cloudy Sunn's 

f Herschel.) fourUrata, 

) photosphere itself. 

e envelope of cloud so often referred to. 

le sun's atmosphere surrounding all, in which the 

Jopes may be supposed to float. 

, who observed the same eclipse, after describing LUtraw's 

lie observations, goes on to say : — opmum. 

cela me fortifie dans Topinion con^ue d6ja par 

vation seule, que ce bord rouge forme une couche 

It toute la photosphere du soleil, et gonfl^e 9^ et 

iberances." * 

n his 'Astronomic'^ defends his first view, and Aragfs 

Ir. Swan's hypothesis. He remarks : — JlTswati's 

s'explique dans Thypoth^se de nuages flottants hypothesis. 

osph^re diaphane qui entoure le soleil. 

1 cherche de rendre compte des protuberances 

\ en les assimilant k des nuages flottants dans 

:re diaphane dont je supposais la photosphere 

M. Swan ayant sans doute remarqui dans ma 
e phrase ' L'^clipse de 1-842 nous a mis sur la 

troisi^me enveloppe situ^e au-dessus de la 
re et form^e de nuages obscurs ou faiblement 
accumule ^ la fin de son m^moire citations sur 
our prouver que nonobstant ce que cette phrase 
enfermer de positif, je n'ai pas eu la pens^e qu'il 
dessus de la photosphere une couche continue de 

E RECONNAIS LOYALEMENT QUE L'ID£E DE LA 
)NTINUE APPARTIENT EN PROPRE X M. SWAN." 

ct criticism we find brings in the corona in 
hall see it again brought forward subsequently, 
ite ext^rieure de la prtinih^e conronne lumineuse 
, dans rhypothise de M. Swan, la r^ion 

chrichten, t. xxxiv., p. 31, and Comptes Rendus^ Feb. 22, 

• Ed, 1856, t iiL p. 623. 

H H 2 




SOLAK PHYSICS. 

qu'occupe la couche continue de nuages dont il croit zvm 
bcsoin pour expliquer tous les phenom^nes des eclipses 
totales. II faudrait done supposer que, lorsquc la couronne 
est unique, cette couche de nuages s'est abaissee jusqu'i 
ttre presque en contact avec la photosphere sotaire, C'cst 
alors qu'apparaitraient Ics longs arcs courbes, colores et 
fortemcnt denteles. qui otit ^t^ signales par les obscrvatcura 
somme etant visibles quclques instants a pre s les conimoice- 
ment dt I'ticlipse totale, et quelques instants avant la Sn. 
Mais admettant pour un moment que ces grands niouvc- 
ments oscillatoires en hauteur de la couche nuageusi: 
existent, pourquot cette couche se preset! te rait -el I c com tiw 
uiie ligne circulaire sans couleur lorsqu'elle scrait k une 
grandc hauteur, et descend rait-e He iris^e et tri;s-irr^gulii« 
dans son contour lorsqu'clle se rapprocherait du soleiL 
Suivant M. Swan, les protuberances sont des portions dt 
son atmosphere continue, soulevces du-dessus du niveau 
general par le coarant ascendant. Mais comment n'a-t-il 
pas reniarque qu'cn 1843 ces protuberances existaient tout 




OBSERVATORY IVORK. 



469 



rnced his report by thus defining the corona and 
s:^ — "La couronne lumineuse ne serait autre 
rindice sensible d'une troisi^me enveloppe du 
5 atmosphere extdrieure. . . . Les protub&ances 
nuages de cette troisiime atmosphh'ey 
lot been able to obtain the CommL<isioner's 

M. Liais, in a separate work,^ states distinctly 
lere is a continuous envelope overlying the 
\^ (II.) that it is not the corona, (HI.) that it is 
>f the general absorption of the photospheric 
[V.) that its height is about 3"*3 — an immense 
ance, as we now know, on the ideas of Arago. 
z time M. Liais was convinced that the corona 
ty a solar appendage. 

ms Spanish eclipse of i860 is next on the list, 
lue's admirable photographs have made us all 
h the solar appendages then visible. Specially 
d in them are the points where the limb of the 
on were nearly in contact both at the commence- 
t the end of the totality. 
^ing the results of this eclipse, both M. Le 

Father Secchi found themselves compelled to 
rd "envelope " to explain all the phenomena 



SECOND 
PAPER. 

Fay/s 

report OH 

Liaiis oIh 

servtUions, 



Liais is 

convinced 

thai the 

corona is 

solar. 



terrier,* after a preliminary discussion of the 
his eclipse, remarked, " Faut il croire que la 
^re de Tastre en (nuages rouges) est parsemie 
faible hauteur comme elle est semde de facules, 
uages roses en sont des Emanations commes les 
ipparaissent sur la disque de I'astre." 
I, after a more complete discussion, he endorses 
the complete continuity of the envelope, and 
the same time not only the only solar atmo- 
the origin of spots ! 

r Rendu s^ vol. xlviii., p. 163. 

ice Celeste et la Nature Tropicale." 

quoted in Comptes Rendus, February 8, 1869, p. 316. 



Le 

Vcrrier's 

discussion 

of the 
eclipse of 

i860. 



and remarking 

on eat d& ajonter uoe 

rcasemble des nuage 

■c h plupart de ces en- 

: que Ic soldi nc soit 

m rsicon de sa hautt 

comAe continue dc la 

ia)c>Brd'hui rexistcncc 

IJquide ou solide. 

bfls la loi CDcnmuiie dc 

.. . nparaltdatrqii'^i 

aeddentclieincnt d'unc 

tovte la surface du soW 

ir AeSiaoaecoades. . . 

to sbow, as we have seen 
'*:si tsTiZ "iL Lmb 4» —y yean before, that the 
r.rT:r Y rrte tib^ b the to tfats envelope, and then 
' - 1.-T i.A=^ dthi 3 wto de I'obseTvation da 
~ -^ a_r?^ ::<; Ja aobirc de fatojosphere s'accumuie 




rxSyT-. i :ji- -_. 




OBSERVATORY WORK, 471 



gradi, ci tnostra che h irragion^vole supporle, particolariti second 
locali ed eccezionali sulla superficie solare, come sono le ^^^^^ 
macchie, n^ possono dirsi eruzioni vulcaniche di pochi 
punti : al contrario il vederle spuntare congiunte in lunghe 
catene tanto al principio che al fine della totalita, ci per- 
suade che negli altri punti della circonferenza si rendono 
visibili solo le cime maggiori e piu elevati, restando le 
minori e pii!i basse coperte dal corpo lunare." 

Although these modern results would seem to have The ton- 
settled the question as far as it was possible to settle it by Ihe^Jip^ 
ordinary observations on the central line of totality, I not yet 
cannot find that the idea of the continuity of the envelope 



generally accepted in England or in France. 

In 1 861 we find M. Faye ^ calling upon observers of the 
eclipse that was to happen on the last day of that year to 
examine "si cette aureole (la partie de cette couronne la 
plus voisine du soleil) prdsente ou non le renversement du 
q>ectre solaire, c'est k dire si les raies obscures de Fraun- 
hofer seront remplac^es dans ce spectre par des raies 
brillantes." There is not one word about the "couche 
rose " of M. Le Verrier ! 

After this in England we find General Sabine, Dr. Sabine, 
Balfour Stewart, and Professor Challis independently ^^J^* 
arriving at the conclusion that the red flames are solar Chains 
theory which I think plainly indicates that the th^hi^red 
that they formed part of a continuous envelope was flames are 
not in their minds. Mr. Balfour Stewart, in a Lecture at ^1^^ 
tiie Royal Institution,* remarked : — 

•* In support of this hypothesis it may be remarked that, 
during the late total eclipse in Spain, Mr. De la Rue, by 
means of the Kew photoheliograph, proved that these red 
flames belong to the sun, and that they extended in one case 
to the distance of 70,000 miles beyond his photosphere. 
But, considering the gravity of the sun, we are naturally 
unwilling to suppose that there can be any considerable 

' Comptes Rendus, vol. lii., p. 679. 
* Proceedings of the Royal Institution^ vol. iv., p. 60. 



aurora. 



OBSERVATORY WORK. 



473 



Broadly, then, up to the spectroscopic determination of 
its continuity and real nature, the story of the chromo- 
q[>here is as follows : — 

1842. Arago refers it to clouds at the base of the 
corona, and regards these clouds as the origin of non- 
nucleated spots, the dimming of the limb being due to the 
corona. 

1842. Grant acknowledges the continuity of the envelope, 
ascribes its brilliancy to reflected light, and thinks it may 
be due to the " cloudy stratum " being driven by convec- 
tion-currents through the photosphere. 

185 1. Swan terms it a new envelope of the sun, states 
that it shines by reflected light, and ascribes to it the 
dimming of the limb. 

185 1. Littrow describes it as a continuous thin envelope. 

1858. Liais describes it as a continuous envelope, and 
gives its thickness as 3"*3. 

i860. Le Verrier describes it as the unique atmo- 
sphere of a solid sun and the origin of all spots. 

i860. Secchi acknowledges it as a third envelope. 

1867. Stoney considers it as a stratum of cloud at a 
distance of 8" or 10" from the photosphere. 



SECOND 
PAPER. 



Gtneral 
remits of 

the pre- 

spectro- 

scopie <^ 

servaiions 

and 

theories. 



Description of the Plates.^ 

Plate XXX VI I , 

Figs. 86, 87, and 88 show the position of the lines observed 
on October 20, 1868, and their usual form — i,e, the line F 
is broad at the base and gradually tapers upwards, while C, 
and the line near D (with no corresponding absorption-line 
ordinarily visible) do not as a rule, present this peculiarity 
with the instrument employed. 

Fig. 91 shows the appearances of the F line observed on 
the 5th of November, 1868. 

Figs. 89 and 100 show the appearances of the C and F 

* The Plates have been distributed throughout the book. See 
chapter xiv. Figs. 84, 85, 86, 87, 88, 89, 91, &c. 



SOLAS PHYSICS. 

lines obi«-\-ed with a tangential slit ; in the case of F 
. the bright line observed was sometimes on one side of 
the absorption -line, and sometimes put it out altogether 
I October 27). 

Figs. 7, 8, and 9' are outlines of the prominences ob- 
seived on the 20th of October and 5th and 6th of Novem- 
ber 1S6S. 



PlaU XXXVIII? 

The spectroscope of large dispersive power, by meins 
of ivhich the work I have described in the former part of 
this paper has been done, is attached to a clock-work- 
driven refractor of 63 inches aperture and 98J iachei 
focal length, the deGnition of which is very fine, anil 
worth)- of the reputation of its Hikers, Messre. Cooke and 
Sons, York. 

Fig, 84 will give a general idea of the spectroscope and of 
its attachment to the large equatorial. In this figure are 
shov^^ the eye-end of the telescope with finder and clarap- 




OBSERVATORY WORK. 47S 

index, and dispersive power, as determined by Mr. Brown- second 
ii^, are as follows : — v^txx. 

Specific gravity 391 

Refractive index 1665 

Dispersive power 0'07S2 

Ordinarily the seven prisms are alone used, but when 
more dispersion is wanted I have found it very convenient 
to fix an extra prism of 60°, as shown in the figure ; this 
makes the instrument for some part of the spectrum a 
direct-vision one ; and 1 have further increased the dis- 
persion by partly filling the small telescope with direct- 
vision prisms. 

The adjustment of the spectroscope to the telescope 
allows of the slit being brought either tangentially or 
radially on any part of the sun's limb. 

Fig, 143 shows a slit I have designed for comparing various Slu/or 
portions of the solar surface with each other. The slit, for ^'^£^^ 




H 



ria 143.— Amntemnii 



purposes of description, may be imagined to be divided 
into three portions. The central portion admits direct light 
from some part of the sun ; the upper and lower portions 
admit light from two other parts of the sun reflected on the 
dit by means of combinations of prisms. 

The slit used in ordinary investigation is similar to that 
usually employed. 

The train of prisms, the collimator, micrometer, and 
observing telescope are shown in Fig. 59 on a larger scale. 
The prisms are fixed to a plate which it is possible to 
rotate slightly on its axis, and this, after the more obvious 



SOLAS PHYSICS. 

precautions have been taken, constitutes one of the most 
. important adjustments. 

The collimator is furnished not only with the usual 
focussing-screw, but with additional rackwork, which permiti 
of the slit being placed in the image, the colour of which 
is coincident with the arc under examination. It is only 
by paying infinite attention to this point that any good 
results can be obtained ; and when this is done, and the 
atmosphere is pure and calm, the interval between each of 
the higher cloud- domes on the sun shows an approach 
to the spoKspectrum, and the spectrum is a mass of hori- 
zontal lines. It is rare, however, that the atmosphere k 
steady enough to show this effect in its greatest perfection. 
The principal point about the micrometer-arrangemcnl 
is, that the micrometer-head is read by a little telescope, 
the eye-end of which is immediately above that of the 
observing telescope ; this saves much time, as the position 
of the obser\^er Is not disturbed when readings are necewar)-. 
I have also found it convenient to supplement this arrange- 
ncnt by an ordinary e\'Cpiecc-micromcter for differential 




OBSERVATORY WORK. 477 



Third Paper. 

Spectroscopic Observations of the Sun. No III. — By J. 
Norman Lockyer. From the Proceedings of the Royal 
Society, No. no, 1869. Received March 4, 1869.* 

Since my second paper under the above title was com- third 
municated to the Royal Society, the weather has been un- ^'^^^^' 
favourable to observatory work to an almost unprecedented 
degree ; and, as a consequence, the number of observations 
I have been enabled to make during the last four months 
is very much smaller than I had hoped it would be. 

Fortunately, however, the time has not been wholly lost 
in consequence of the weather; for, by the kindness of 
Dr. Frankland, I have been able in the interim to familiar- 
ize myself at the Royal College of Chemistry with the 
spectra of gases and vapours under previously untried 
conditions, and, in addition to the results already com- 
municated to the Royal Society by Dr. Frankland and 
myself, the experience I have gained at the College of 
Chemistry has guided me greatly in my observations at 
the telescope. 

In my former paper it was stated that a diligent search 
after the known third line of hydrogen in the spectrum 
of the chromosphere had not met with success. When, Dtsawery 
however, Dr. Frankland and myself had determined that <^fj^^^^ 
the pressure in the chromosphere even was small, and line in the 
that the widening out of the hydrogen lines was due in ^^^j^' 
the main, if not entirely, to pressure, I determined to seek (H^y). 
for it again under better atmospheric conditions; and I 
succeeded after some failures. The position of this third 
line is at 2796 of Kirchhoff's scale. It is generally exces- 
sively faint, and much more care is required to see it than 
is necessary in the case of the other lines ; the least haze 
in the sky puts it out altogether. 

Hence, then, with the exception of the bright yellow 

* Proc. R.S.y vol. xvii. p. 350. 




Dr. Frankland and 

fae 4Be to the radiation 

A becxme a matter of 

Bae Ac mi and green 

oa to the limb. 

in my instrument 

C absolutely on the 

m it 

pooited oat that. 

Ac pKMoiDCDces give out 

follow that they are 

: sspposiog otben to 

presume that they 

absorptioD near the 

*^gij!jijni pf the limb would 




OBSERVATORY WORK. 



479 



III. That it is not probable that the prominences will 
be visible on the sun's disc. 

In connection with the probable chromospheric darken- 
ing of the limb, an observation of a spot on February 
20th is of importance. The spot observed was near the 
limb, and the absorption was much greater than anything 
I had seen before ; so great, in fact, was the general absorp- 
tion, that the several lines could only be distinguished with 
difficulty, except in the very brightest region. I ascribe 
this to the greater length of the absorbing medium in the 
spot itself in the line of sight, when the spot is observed 
near the limb, than when it is observed in the centre of 
the disc — another indication of the great general absorb- 
ing power of a comparatively thin layer, on rays passing 
through it obliquely. 

I now come to the selective absorption in a spot. I have 
commenced a map of the spot-spectrum, which, however, 
mrill require some time to complete. In the interim, I may 
state that the result of my work up to the present time 
in this direction has been to add magnesium and barium 
to the material (sodium) to which I referred in my paper 
in 1866, No. I. of the present series; and I no longer 
regard a spot simply as a cavity, but as a place in which 
principally the vapours of sodium, barium and magnesium 
(owing to a downrush) occupy a lower position than they 
do ordinarily in the photosphere. 

I do not make this assertion merely on the strength of 
the lines observed to be thickest in the spot-spectrum, but 
also upon the following observations on the chromosphere 
made on the 2 fst and 2Sth ultimo. 

On both these days the brilliancy of the F line taught 
me that something tinusual was going on ; so I swept along 
the spectrum to see if any materials were being injected 
into the chromosphere. 

On the 2 1st I caught a trace of magnesium ; but it was 
late in the day, and I was compelled to cease observing by 
houses hiding the sun. 



THIRD 
PAPER. 

Chronuh 

spheric 

darkening 

of limb. 



The spot" 
spectrum. 



H^hata 
spot is. 



SOtAM pffystcs. 

Zt rzjt ;ki I was aaore romuute. If anything, the 
, jr-tzc3cs5 c< gier.^ actBDa were stronger than on the 2ist, 
5^ iTi^ cifx g^>i3ae aC die F tiae I turned at once Xa 
t^e aag -a s:^ji l;:;es. I bv them appearing short and 
i-^n ii lb; "r.i>e c4 tlie dromosphcre. My work on the 
^vcs i=i =e r J .:::^giiie that I should find sodiura-\'apour 
j^ss-x-.-.TiK. w-iiii the iB^oeaum ; and on turning from h 
t_- r I >>:i^- ii:--s to be the ca^e. I afterwards reversed 
zin-Li^ iz thi; same way. TTie spcctnim of the chroRio- 
-rirers secri'sd lo be fJH of lines, and I do not think the 
zrr^ s::bsc£r>:ss I ha»e named accounted for all of them. 
Tbe .\:ser\aiioG was one of excessive delicacy, as Che lines 
Terr fccvt 2^z rtry dttm. The prominence was a small 
oc^ >>--u: rxic£ the usual height of the chromosphere; 
":_i ize hvi:-.-^en Uncs towered high above those due to the 
T<eiiv ^-"sctffi niitenal^ The lines of magnesium extended 
r<T^=s cce-sixth of the height of the F line, barium 3 
::-j >S5. a=^i s>iium least of all. 
■A^; >:v;. tlie'. the following facts: — 



obse'rvatory work. 



481 



forward the theory of a downnish about the same time 
as my observations were made in 1865, at once suggested 
as one advantage of this explanation that all the grada- 
tions of darkness, from the faculae to the central umbra, are 
thus supposed to be due to the same cause, namely, the 
presence to a greater or less extent of a relatively cooler 
absorbing atmosphere. This I think is now spectroscopic 
cally established; we have, in fact, two causes for the 
darkening of a spot : — 

I. The general absorption of the chromosphere, thicker 
here than elsewhere, as the spot is a cavity. 

II. The greater selective absorption of the lower sodium, 
barium, magnesium stratum, the surface of its last layer 
being below the ordinary level 

Messrs. De la Rue, Stewart, and Loewy also suggested, 
in their " Researches on Solar Physics," that if the photo- 
sphere of the sun be the plane of condensation of gaseous 
matter, the plane may be found to be subject to periodical 
elevations and depressions, and that at the epoch of mini- 
mum sun-spot frequency the plane might be uplifted very 
high in the solar atmosphere, so that there was compara- 
tively little cold absorbing atmosphere above it, and there- 
fore great difficulty in forming a spot 

This suggestion is one of great value ; and, as I pointed 
out in my previous paper, its accuracy can fortunately now 
be tested. It may happen, however, that in similar 
periodical fluctuations the chromosphere may be carried up 
and down with the photosphere ; and I have already 
evidence that possibly such a state of things may have 
occurred since i860, for I do not find the C and F Fraun- 
hofer lines of the same relative thickness as they were in 
that year.^ I am waiting to make observations with the 
kirge Steinheil spectroscope before I consider this question 



THIRD 
PAPER. 



Causes of 
the dark* 
ness of a 
spot. 



Possible 

fluctuG' 

tions of the 

plane of 

condensa- 

tion. 



Changes in 

the rdatrt'e 

thickness of 

C and F. 



1 I have learnt, after handing this paper in to the Royal Society, 
that in Angstr5m's map the C and F lines arc nearly of the same 
breadth : this I had gathered from observations made with my own 
spectroscope. 



T T 



saiJx rnrstcs. 



B gnat thtdcness of the F lia<: 
. 5: Sjr^^i iaii ;ti=r i^is wtD potnt out the cxccssire im- 
rur^z^r^ Z.C sxi ibs^raOieas as a metbod of asccrtaiiuog 
T'l'T Id-— Ts^ zcrr^.-z3l ccHSdtatioa, but the actual pressures 
. L ^1= : it^ 1t;.:zs oc aCdlu' Atmospberes, and of the same | 
zz=*-'~;ixrz ■'. z - tr att epoAa. And when other spectra 
z^ . :<-= Kr^i>?d 21 ■« ha\-e now studied hydrogeo. 
i^-z . --i- rzi^iz^ ~f motipaini; siaiilar researches will beit 

- .r ' > .-->-" Izde^ a Mxncu'hat careful examinatioD of 

'_2rf ^ecTTi :/ Lhe difl eiq tt dasses of stars, as defined ty 
T-izn:: xcctl leads ae to believe that several bnxiil 
^tsc^s^:^:^ £r? i>^ ^ to seek ; aod I hope soon to lay I 
z^r=i rtf jre z=< Rm-al Sobet)-. , 

F:r sjrrse r":=e past I have been engaged in endeavour I 
^ ;^ cKiir. a agfai of the prominences, by using a vaj' , 
r£f ^y rtjo-lisnj slit ; bot although I belie\-e this method 
v-^ ^. uTT^^W succeed, the spectroscope 1 emptoy doc 
' : i- ^ - :rc :,:■ ipply it under sufficiently good conditions 
.-11 i.- -_: ^: present satisfied witli the results 1 have 



OBSERVATORY WORK. 4F3 

this method the smallest details of the prominences and third 
of the chromosphere itself are rendered perfectly visible '^^^^^' 
and easy of observation.^ 

Addendum. — Received March 17, 1869. 

Since the foregoing paper was written, I have had, thanks 
to the somewhat better weather, some favourable oppor- 
tunities for continuing two of the lines of research more 
especially alluded to in it ; I refer to the method I had 
adopted for viewing the prominences, and to the injection 
of sodium, magnesium, &c. into the chromosphere. 

With regard to seeing the prominences, I find that, when 
the sky is free from haze, the views I obtain of them are so 
perfect that I have not thought it worth while to remount 
the oscillating slit. I am, however, collecting red and 
green and violet glass, of the required absorptions, to con- 
struct a rapidly revolving wheel, in which the percentages 
of light of each colour may be regulated. In this way I 
think it possible that we may in time be able to see the 
prominences as they really are seen in an eclipse, with the 
additional advantage that we shall be able to see the sun at 
the same time, and test the connection or otherwise between 
the prominences and the surface-phenomena. 

Although I find it generally best for sketching purposes 
to have the open slit in a radial direction, I have lately 
placed it at a tangent to the limb, in order to study the 
general outline of the chromosphere, which in a previous 
communication I stated to be pretty uniform, while M. character 
Janssen has characterized it as " d niveau fort indgal et ofthechro- 
tourment^y My opinion is now that perhaps the mean of 
these two descriptions is, as usual, nearer the truth, unless 
the surface changes its character to a large extent from 
time to time. I find, too, that in different parts the outline 
varies: here it is undulating and billowy; there it is ragged 
to a degree, flames, as it were, darting out of the gf^neral 

' See Note D. 

I I 2 



^ 




I -rt^r^ ' los art M afl faBiw that the bugest 

I JBSC IB, orsbove^ 
• from violeot apnA 




OBSERVATORY WORK, 485 

round 90° and narrowed the slit, and my attention was at third 
once taken by the F line ; a single look at it taught me that ^^'*^'^' 
an injection into the chromosphere and intense action were 
taking place. These phenomena I will refer to subse- 
quently. 

At 10** 50°*, when the action was slackening, I opened A promi- 
the slit ; I saw at once that the dense appearance had all observed to 
disappeared, and cloud-like filaments had taken its place, ^^j^g^ i^s 
The first sketch, embracing an irregular prominence with a * 

long, perfectly straight one, which I called A, was finished at 
11** 5", the height of the prominence being i' 5", or about 
27,000 miles. I left the Observatory for a fev/ minutes ; 
and on returning, at 11** 15", I was astonished to find that 
part of the prominence A had entirely disappeared ; not 
even the slightest track appeared in its place : whether it 
was entirely dissipated, or whether parts of it had been 
wafted towards the other part, I do not know, although 
I think the latter explanation the more probable one, as 
the other part had increased. 

We now come to the other attendant phenomena. First, Behaviour 
as to the F line. In my second paper, under the above ifj^^^^ax 
title, 1 stated that the F line widens as the sun is ap- 
proached, and that sometimes the bright line seems to 
extend on to the sun itself, sometimes on one side of the 
F line, sometimes on the other. 

Dr. Frankland and myself have pointed out, as a result 
of a long series of experiments, that the widening out 
is due to pressure, and apparently not to temperature 
fier se;^ the F line near the vacuum-point is thin, and it 
widens out on both sides (I do not say to the same ex- 
tent) as the pressure is increased. Now, in the absence 
of any disturbing cause, it would appear that when 
the wider line shows itself on the sun on one side of 
the F line, it should at the same time show itself on 
the other ; this, however, it docs not always do. I have 
now additional evidence to adduce on this point, and 

^ See the Laboratory work further on. 



-t~ tTTim^-r.-- 




"Sme &self, ofT the sun. 
I hove refemd, the F 
t ttcaagc contortiMU, 2s 
cutse vliicfa varied the 
Ebc onder csrtam coodi- 



?t also once bore a simiUr 



a which accompanied 
ri=^ xt^ o:cr of Ae F lae. and were apparditiy 
*se It' x^ 

=ie Q-^' ^cU of vicK widi F, I recognized die 
: -im -: ;c.>59^ of Kmttoff's scale 
i=-z --^ t2>e ^asBenoB fines and the inctoud 
-r:n .JK^ Tcae ifaMr ia tie dunmospberc The 
^^= 's-^ ^lifAlul h^ber into the chromosphere 
11-. :.Lr.L:u xad iIk Bidcel or iron n-as projcciol 
'.-.L- zi<i taafpte^Bon. I carefully cxamineii 
^- -^z -r^r iraa litr were visible in the spectrum 




OBSERVATORY WORK. 487 

uprush a dense prominence ; accompanying the uprush we third 
have changes of an enormous magnitude in the prominence ; -Zl!!f^ 
and as the uprush ceases the prominence melts away. 

As stated in the former part of this paper, the barium 
and magnesium lines were thinner than the corresponding 
Fraunhofer lines. In connection with this subject, I beg to 
be allowed to state that I have commenced a careful com- 
parison of KirchhoflTs map with the recently published 
one of Angstrom. From what I have already seen. I 
believe other important conclusions, in addition to that 
before alluded to, may be derived from this comparison ; 
but I hesitate to say more at present, as I have not yet 
been able to compare Angstrom's maps with the sun itself, 
or to examine the angular diameters of the sun registered 
at Greenwich during the present century. 

On the 14th inst. I also succeeded in detecting the Fourth 
hydrogen line in the extreme violet in the spectrum of the iine7fund 
chromosphere. 



SOLAS POYS/CS., 



Fourth Paper. 

Spfctroscopic ObservalioHS of the Sun. No. IV. — By J. 
NoKMAN LoCKVER, F.R.A.S. From the Proceedings of 
the Royal Society, No. Ill, 1869. Received April 14. 

18G9.' 

I BEG to lay berore the Royal Society very briefly the 
results of observations made on the iith instant in the 
neighbourhood of a fine spot, situated not very far from 
the sun's limb. 

I. Under certain conditions the C and F lines may be 
obBcr\cd bright on the sun? and in the spot -spectrum also, 
as in promiin:nccs or in the chromosphere. 

II. Under certain conditions, although they arc not 
observed as bright lines, the corresponding Fraunhofcr 
liuL-s are blotted out. 

'" III. The accompanying changes of refrangibility of the 
, linos in question show that the absorbing material moves 




OBSERVATORY WORK. 489 

Addendum. — Received April 29, 1869. 

Since the date on which the foregoing paper was written, fourth 
I have obtained additional evidence on the points referred ^^^'^ 
ta I beg therefore to be permitted to make the following 
additions to it. 

The possibility of our being able to determine the 
velocity of movements of uprush and downrush taking 
place in the chromosphere depends upon the alterations of 
wave-length observed. 

It is clear therefore that a mere uprush or downrush at 
the sun*s limb will not affect the wave-length, but that if 
we have at the limb cyclones, or backward or forward 
movements, the wave-length will be altered ; so that we 
may have : — 

I. An alteration of wave-length near the centre of the Upward 

j» ji_ jj J A. and dowtt' 

disc, caused by upward or downward movements. ^^^^^ 

II. An alteration of wave-length close to the limb, caused fnovements 
by backward or forward movements. the centre^ 

If the hydrogen-lines were invariably observed to °^^^^ 
broaden out on both sides, the idea of movement would forward 
require to be received with great caution : we might be in ,!^^i^\ 

^ , , ^ ' , , t hmbof the 

presence of phenomena due to greater pressure, both when sun, 
the lines observed are bright or black upon the sun ; but 
when they widen out sometimes on one side, sometimes on 
the other, and sometimes on both, this explanation appears 
to be untenable, as Dr. Frankland and myself in our 
researches at the College of Chemistry have never failed to 
observe a widening out on both sides the F line when the 
pressure of the gas has been increased. 

On the 2 1 St I was enabled to extend my former ob- 
servations. 

On that day the spot, observations of which form the 
subject of the paper, was very near the limb ; as this was 
the first opportunity of observing a fine spot under such 
circumstances I had been able to utilize, I at once com- 
menced work upon it. The spot was so near the limb that 



sooiK pjn'sics. 

its spectrum and that of the chrotnospherc were bcth 

visible in tlie field of view. 

The spot-spectrum was ver>' narrow, as the spot itstU 
was so greatly foreshortened; but Uie spectrum of the 
chromosphere showed mc that the whole adjacent limb 
was covered with prominences of various heights ait 
blended tofiethcr. 

Further, the prominences seemed fed, so to speak, frons, 
apparenti}-, the preceding edge of the spot ; for both C, F, 
and the line near D, uvre magnificently bright an tht tm 
itself, the latter especially striking mc with its thkkness 
and brilliancy. 

In the prominences C and F were observed lo be 
strangely gnarled, knotty, and irregular, and I thought at 
once that somu ■■ injection " must be taking place. I was 
not mistaken. On turning to the magnesium lines 1 saw 
them far above the spectrum of the limb and unconnected 




OBSERVATORY WORK. 491 





permitted to recall the observation made on March 14th, fourth 
in which a slight movement of the slit gave me first ^^'*^'^' 

then I , and finally I ;— all 

Fig. 145. 

these appearances being due to cyclonic action. 

On the following side of the spot, at about 10 A.M., I 
observed that the F line had disappeared ; at the point of 
disappearance there appeared to be an elongated, brilliantly 
illuminated lozenge lying across it at right angles, as if 
the spectroscope were analysing the light proceeding from 
a cyclone of hydrogen on the sun itself, but so near the 
limb that the rotatory motion could be detected. 

The next observations I have to lay before the Royal 
Society were made on the 27th inst. Careful observations 
on the 25 th and 26th revealed nothing remarkable except 
that the chromosphere was unusually uniform. 

On the 27th a fine spot with a long train of smaller ones 
and faculae was well on the disc. The photosphere in 
advance of the spot, and the large spot itself, showed no 
alteration from the usual appearance of the hydrogen- 
lines ; but in the tails of the spot the case was widely 
diflferent. 

The F line, at which I worked generally, as the changes 
of wave-length are better seen, was as irregular as on the 
former occasions. 

I. It often stopped short of one of the small spots, 
swelling out prior to disappearance. 

II. It was invisible in a facula between two small spots. 

III. // was cluinged into a bright line, and widened out V widened 
on both sides two or three times IN THE VERY SMALL SPOTS. /'"^ , 

IV. Once I observed it to become bright near a spot, into a 
and to expand over it on both sides. 

V. Very many times near a spot it widened out, some- 
times considerably, on the less refrangible side. 



bright line 
on a s/'ot. 



i a fei^B^ fine n-hhout ; 



it&ier^ :_. 3 E »i: 



: ■■ bright. 

of diAness. 
wen: sloDgside. the 



4 




OBSERVATORY WORK. 493 



Fifth Paper. 

Spectroscopic Observations of the Sun. No. V. — By J. 
Norman Lockyer, F.R.S. From the Proceedings of 
the Royal Society, No. 115, 1869. Received July 8, 
1869.1 

Since the date of my last communication under the above firth 
title, the weather has, if possible, been worse for telescopic ^^^^^ 
work than during the winter and spring ; my opportunities 
of observation, therefore, have been very limited ; still the 
sun has occasionally been in such a disturbed state, and 
our atmosphere has at times been so pure, that several new 
facts of importance have come out. 

I will state them here as briefly as possible, reserving a 
discussion of them and my detailed observations for a 
future occasion. 

I. The extreme rates of movement in the chromosphere RaUsof 
observed up to the present time are : — movement 

t^ *^ xn cyclones. 

Vertical movement 40 miles a second. 

Horizontal or cyclonic movement. 120 „ 

II. I have carefully observed the chromosphere when 
spots have been near the limb. The spots have sometimes 
been accompanied by prominences, at other times they 
have not been so accompanied. Such observations show 
that we may have spots visible without prominences in the 
same region, and prominences without spots ; but I do 
not say that a spot is not accompanied by a prominence 
at some stage of its life, or that it does not result from 
some action which, in the majority of cases, is accompanied 
by a prominence. 

III. At times, when a prominence is seen bright on 
the sun itself, the bright F line varies considerably, both 
in thickness and brilliancy, within the thickness of the 
dark line. The appearances presented are exactly as if 
we were looking at the prominences through a grating. 

* Proc. R,S,, vol. xviii. pp. 74 and 118. 



SOLAR PHYSiCS. 

IV. Bright prominences, when seen above spots on the 
Liijc. li built up of other substances besides hydrogen, 
are indicated by the bright lines of those substances in 
addition to the lines of hydrogen. The bright lines are 
Then seen verj- ihin, situated centrally {or nearly so) on the 
b!-?.id absorption- hands caused by the underlying Ies<^- 
I'^mincus vapourji of the same substances. 

V I have at last detected an absorption-line correspond- 
, \vi^ to the orange line in the chromosphere. Father 
^ecchi states' that there is a line corresponding to it mudi 
bri^bter than the rest of the spectrum. My observation 
would seem to indicate that he has observed a br^t 
lice less refrangible than the one in question, uhich bright 
iiiv is .11 times excessively briUiant. It requires abso- 
liiloly perfect atmospheric conditions to see it in the 
•.■rdinarj- solar spectrum. It is best seen in a spot- 
spectnini when the spot is partially covered by a bright 



ghbourhood of spots the F bright Hoc 




OBSERVATORY WORK. 495 



shows traces of absorption, which gradually diminish as the fifth 
higher strata of the chromosphere are brought on to the ^^^^^' 
slit, until the absorption-line finally thins out and entirely 
disappears. The lines of other substances thus observed 
do not show this absorption. 

X. During the most recent observations I have been 
able to detect traces of magnesium and iron in nearly all 
solar latitudes in the chromosphere. If this be not merely 
the result of the good definition lately, it would indicate 
an increased general photospheric disturbance as the maxi- 
mum sun-spot period is approached. Moreover I suspect 
that the chromosphere has lost somewhat of its height. 

I append a list of the bright lines,^ the positions of 
which in the chromosphere I have determined absolutely, 
with the dates of discovery, remarking that in the case 
of C and F my observations were anticipated by M. 
Janssen : — 

Hydrogen. List oj 

chromo- 
C. October 20, 1868. spheric 

F. October 20, 1 868 ^X'^'^^ 

near D. October 20, 1868.^ 

near G. December 22, 1868. 

//. March 14, 1869. 

Sodium. 
D. February 28, 1869. 

Barium. 

19895.'^ March 14, 1869. 
2031*2. July 5, 1869. 

Magnesium and included line. 
^Y February 21, 1869. 

^ See Note H. -' Hydrogen t 

^ This reference is to Kirchhoff's scale. 



souta pxnrsscs. 



OtikrLma, 

Irrm. ..... 1474.* JmMtS, r869. 

? I5IS-S. Jane6,iSS9^ 

Bright Mm . - \yB5f^ July 5- »*fi» 

^ i^TS- Uarch %. 1M9. 

' l6i3'& Jnoe & 

Iron ...... 1867^1 jone 26. 

Bright a*. . 1871-5. 

Iroa 2CX>i-5. V 

• band or line ri^ar bfadc ) ^. . . 

line. ver>- delicate ... J "*'**'* ■''"J^ ^ 

I have seen othef Uses besides these at different timo. 
but I do not include tlietn, as their positions have not beu 
d'^termined absolutely. 

I refrain from dwdEng o«i the list at present, except to 
f> lint out that, taking iroa as an instance, and asaummg 
that the iron line; mapped bjr An^trom and Kirchhoffsc 
due to iron r,n'.;.-, T have only been able, up to the preenl 




OBSERVATORY WORK. 497 



and the photosphere form the true atmosphere of the sun, fifih 
and that under ordinary circumstances the absorption is ^^^^^ 
continuous from the top of the chromosphere to the bottom 
of the photosphere, at whatever depth from the bottom of 
the spot that bottom may be assumed to be. 

This theory was based upon all our observations made 
from 1866 up to the time at which it was communicated to 
the Royal Society and the Paris Academy of Sciences, and 
has been strengthened by all our subsequent work ; but 
several announcements made by Father Secchi to the Paris 
Academy of Sciences and other learned bodies are so 
opposed to it, and differ so much from my own observa- 
tions, that it is necessary that I should refer to them, and 
give my reasons for still thinking that the theory above 
referred to is not in disaccord with facts. At the same 
time I must state that Father Secchi does not combat this 
theory; indeed it is not to be gathered from any of his 
communications that he has seen any of the papers com- 
municated by myself to the Royal Society. 

Father Secchi states that the chromosphere is often SecckPs 
separated from the photosphere, and that between the *^'^'^^'- 
chromosphere and the photosphere there exists a stratum 
giving a continuous spectrum, which he considers to be the 
base of the solar atmosphere, and in which he thinks that 
the inversion of the spectrum takes place.^ 

With regard to the first assertion, I may first state that 
all the observations I have made have led me to a contrary 
conclusion. Secondly, in an instrument of comparatively 
small dispersive power, such as that employed by Father 
Secchi, in which the widening out of the F line at the base 
of the chromosphere is not clearly indicated, it is almost 
impossible to determine, by means of the spectroscope, 
whether the chromosphere rests on the sun or not, as the 
chromosphere is an envelope and we are not dealing merely 
with a section. But an instrument of great dispersive 
power can at once settle the question ; for since the F 

I See Note K. 

K K 



SOLAK PHYSta. 

line widens ciut witli pressure, and as the pressure h- 
creases as the sun is approached, the continuous cuTvatm 
of the F Hne must indicate really the spectrum of i 
section ; and if the chromosphere were suspended merely 
at a certain height above the photosphere, we should noi 
get a widening due to pressure: but we always do gn 
such a widening. 

With regard to the second assertion, I would remark 
that if such a continiious-spectrum-giviiig envelope existed, 
I entirely fail to see how it could be regarded as a region 
of selective absorption. Secondly, my observations have 
indicated no such stratum, although injections of sodiua 
magnesium, &c. into the chromosphere not exceeding the 
limit of the sun's limb by 2" have been regularly observed 
for several months past To-day I have even detected i 
low level of barium in tlie chromosphere not l" h^h, 
This indicates, I think, that my instrument is not lacJcing 
in delicacy ; and as I have never seen anything approach- 
ing to a continuous spectrum when my instrument ki 
lii.L-n ill iierfiL'ct ndju^tmcnt. 1 am inclined to attribute ih; 



OBSERVATORY WORK. 499 

Father Secchi remarks also that the F line is produced fifth 
by the absorption of other bodies besides hydrogen, ^^^^^ 
because it never disappears. This conclusion is also 
negatived by my observations ; for it has very often 
been observed to disappear altogether and to be replaced 
by a bright line. At times, as I pointed out to the Royal 
Society some months ago, when a violent storm is going 
on accompanied by rapid elevations and depressions of 
the prominences, there is a black line on the less refrang- a Mack 
ible side of the bright one ; but this is a phenomenon due ^^^j^ jf/ 
to a change of wave-length caused by the rapid motion of side of Y. 
the hydrogen. 

With regard to the observation of spot-spectra, I find 
that every increase of dispersive power renders the pheno- 
menon much more clear, and at the same time more 
simple. The selective absorption I discovered in 1866 
comes out in its most intense form, but without any of the 
more complicated accompaniments described by Father 
Secchi. I find, however, that by using three prisms this 
simplicity vanishes to a great extent. We get portions 
of the spectrum here and there abnormally bright, which 
have given rise doubtless to some of the statements of the 
distinguished Roman observer ; but the bright lines, pro- 
perly so-called, are as variable as they are in any other part 
of the disc, but not much more so. I quite agree that the 
" interpretation '* of sun-spot phenomena to which Father 
Secchi has referred,^ which ascribes the appearances to 
anything but selective plus general absorption, is erroneous. 
But as I was not aware that it had ever been propounded, 
I can only refer to my own prior papers in support of my 
assertion, and to Mr. Huggins' endorsement of my obser- 
vations, which were communicated to the Royal Society 
some three years ago. 

1 Comptes Rendus, 1869, ir. sem. p. 764. I have since found that 
Father Secchi was combating some statements made by Professor 
Respighi, whose paper I had not then seen. 

K K 2 



SOLAS PHYSICS. 

t This paper, which appeared in the Comptes Rctidm of 

'■■ _ the French Academy, as well as in the Proceedings of tlit 
Rnyal Socicly, was followed by a discussion between Father 
Sccchi and myself, in the Comptes Rendus of the Paris 
Academy, which, in justice to that obseirer, must be given 
here. Father Secchi's reply' runs as follows : — 

" Rome, ct lajuilUt, 1869. 
"Absent de I'Observatoire pendant quelques jours, c'est 
sculemcnt hicr que j'ai eu connaissance de la Note dc M. 
Loclcyer, inseree dans les Comptes Rendus du I2 juillct 
Uans cctte Note, M. Lockyer repr^ente nies resultats sur 
la constitution du Soleil comme trts-opposh atix suns, mais 
il avoue en meme temps que j'e ite combats pas sa th/orU; 
il en coiiciut que ;V n'ai pas connaissatice des Mfmmrts 
prcscnlvs par liii a la Soci^t^ Royale. Pour bien jugcr tie 
cette divergence, il faut s^parer deux choses, les fails les 
hypotheses. 1° M. Lockyer conteste des faits observes par 
moi : jc fcrai done voir quelle est la cause pour laquellc il 
ii'a pas pu les verifier ; 2''il expose une theorie sur iaqutUc 
nes partiellemcnt d'accord, et que je rejettc e 




OBSERVATORY WORK. 501 

ligne du magnesium est renvers^e^ et que V autre ligne brillante fifth 
occupe V espace intermidiaire des deux plus voisines, Ce sont ^'^'*^^- 



des faits capitaux dans notre sujet, qu'on ne peut pas Only one 
^noncer d'une mani^re aussi vague que le fait M. Lockyer. ^^^^ ^/ 

* t /.-»«■ -r 1 magnesium 

11 est done grandement a desirer que M. Lockyer com- reversed, 
munique ses r^sultats avec plus de d^tails^ pour ^tablir ses 
droits sur ce point et sur les autres. 

"Venons maintenant aux faits qu'il met en doute. II 
conteste d'abord ce que j'ai dit sur la couche k spectre ihe 
continu que j'ai viie dans le Soleil, entre le bord et la *^^^g^ 
chromosphere ; et, dans sa critique, il m^le des id6es theo- continue 
riques que je n'accepte pas. Quant au fait, je Tai assez 
souvent vu et revu pour n*en pouvoir pas douter ; j'ai meme 
d^taill^ toutes les circonstances dans lesquelles il se mani- 
feste et les precautions k employer pour le voir, de sorte que 
je n'ai aucun doute, et je n'ai pas k revenir sur ce point. 
Dans cette mati^re, une observation negative ne fait pas 
autorit^ et ce sont ces r^sultats n6gatifs qui ont tant 
retard^ la d^couverte de ces ph^nom^nes si faciles k voir. 

" M. Lockyer rejette son insucc^s sur la petitesse de mon 
instrument : dans une autre question parall^le k celle-ci, 
on a aussi commence par cette objection, mais on a fini jTie p<rwer 
par admettre le resultat : il en sera ainsi de la pr6sente ; ^"^^f^''^ 
car un spectroscope qui fait voir toutes les raies de Kirch- ment. 
hoff, et qui possMe une telle dispersion et une telle force 
refractive, que le rayon sorti des trois prismes est parall^le 
au rayon entrant, un instrument auquel on peut appliquer 
de plus un prisme k vision directe de la force de deux 
autres prismes, de mani^re k en faire en reality un instru- 
ment k cinq prismes, ne me paralt pas un instrument faible 
pour cette espice de recherches. 

" Je crois, au contraire, que le d^faut est du c6t^ de M. Necessity of 
Lockyer. Je ne connais pas en detail sa mani^re d'obser- ^*",^^ 
vcr, ni son instrument, mais il me semble, par la description of the Suv. 
de ses r^sultats, qu'il ne grossit pas, ou qu'il grossit peu 
Timage du Soleil. Dans ce cas, il est evident qu*il ne 
pourra pas sparer la lumiere de la couche en question de 



SOLAK fHYStCS. 



IfVM , 



la lumiire de couches qui renvironncnt, car cette couche 
aura a peine lepaisscur de la largeur de la fente dans 
linia^e dirccte dune lunette de oeuf pouces, comoie U 
miennc, Au contraire. avec mon systems d'obscrvatioa. 
en ;;;r05iissant coiivenablement I'image directe, on donnc 
plus de lai^eur a la couche, et il devient possible d« li 
separcr dts autres. 

" Jc crois encore que M. Lockj-er se mcprcad, lorequll 
dit que mon instrument est incapable delargir la raic F i 
la base, et que cette m^prise ticat d la mdmc caas&l) 
petitesse de son image directe. En effet, en emplo)^itf 
I'iniage directe, j'ai va la raie F tr^-brillante ct en forme 
de fer de lance, et en ouvrant un peu la fente on voyaJt 
toute la protuberance, et on rele\-ail son contour ordinaire- 
,'"' ment conique. Mais ces apparences s'evanouissent ca 
'. grossissant I'image solaire, car la protuberance acquis 
alors unc hauteur lineaire plus grande, et line largcur qui 
surpasse plusicurs fois la larg'eur de la fente. 

" M. Lockyer insiste beaucoup sur lelargtsscmait de 




OBSERVATORY WORK. 



503 



TigJlJ 



TiyS^ 



I 



d'une difference de refrangibilite, qui, ajoutant de nouveaux 
rayons a droite et a gauche de la raie, I'^largit aussi. Ces 
deux causes sont egalement possibles et probables, et il 
reste k trouver la veritable. 

•* C'est d abord un fait bien constate que I'lntensit^ de la 
lumiere des protuberances n*est pas toujours la m^me et 
que Tirradiation est parfois assez forte pour donner k la 

raie C la forme de coin paraissant plus dilatde 
i li ou elle est plus vive : mais, en mettant 
^^A^ soigneusement au point focal de vision la fente, 
et surtout en amplifiant Timage de la protu- 
berance, comme je le fais habituellement, on trouve tou- 
jours une portion rectiligne termin^e par une pointe. Et 

s*il y a un nuage suspendu, on voit nette- 
ment le milieu du nuage rectiligne et les 
extr^mitds en pointe effilde. Cette pointe, 
du reste, peut bien s'expliquer, soit par la 
density du nuage soit par I'intensite de la 
lumiere. 

"Je crois done que, si M. Lockyer grossit convenable- 
ment ses images, il verra disparaitre ces courbures des- 
quelles il tire beaucoup de conclusions, qui sont pour cette 
raison au moins douteuses, et qui pourraient bien etre, 
ou un effet de la forme meme de la protuberance qui 
serait plus ^troite que la fente elle-meme, ou un effet 
d'irradiation. 

" M. Lockyer continue ses objections par des considera- 
tions th^oriques, et dit que, si la chromosphere iftait suspen- 
due i une certaine distance delaphotosphtrCy nous ne ponrrions 
trouver un dargissement dA a la prcssion, J'avoue que je 
ne vois pas la legitimit^ de cette conclusion ; car, m^me en 
admettant la chromosphere suspendue, elle devrait toujours 
suivre la loi du decroissement de densite que subit Tatmo- 
sph^re solaire dans laquelle elle nage. II faut bien remar- 
quer que cette structure des masses suspendues dans une 
atmosphere ne r^sulte pas des observations spectroscopiques, 
mais bien des observations des Eclipses ; et il est impossible 



FIFTH 
PAPER. 



Appear- 
ance of C 



The objec- 
tions to the 
** couche a 

spectre 

conliftH ** 

im*alid. 



SOLAR PHYSICS. 



d'admettre que ces nuages ou ces colonnes iadinees puis- 
sent rester suspetidues, sans un milieu qui les supporte et 
qui soit different d'elles-itifimes, SL Lockyer, qui n admct 
J pas ce milieu, oil nagent les protuberances, trouve saas 
- doute inadmissibles bicn des choses, maU a notre tour 
nous n'admettons pas son hypothese, que la chromospkirt 
soil la di-rnU-re coitcke de ratmospkire solatre. 

■■ Mais laissant de c6t^ la th^orie et revenant aux faits, 
il me semble que, pendant que M. Lockj-er rejette mcs 
rtJsultats, il vient reellcment les appuyer par ses obscr\-a- 
tions. En efftt, il dit avoir vu pattout de norabreuse 
emanations de sodium de 1 4 2 secondes, et d'autics 
nii-taux', etc. Or, Je demaode comment il a vu ces Emana- 
tions? Sans doute par le renversement des raies ou par 
Vabscnce des raies noires : or c'cst \k pr^cisement le fait 
contestc; c'est-a-dire qu'il y a au bord du Soleil un filcl 
tres-mince, oil un grand noitibre de ces raies et parfois 
toutes les plus faibles dUparaissent. M. Lockj'er appuie 
done mon observation en la combattant, et la seule diffe- 
rence entre nous serait qu'il a eu occasion de voir des phino- 




OBSERVATORY WORK. 505 



P. Secchi soient des arguments difinitifs contre une partie de fifth 
la thiorie que je viens de rappeler, etc. Autant qu'il m'est _I^^1_ 
possible de comprendre cette phrase, nous ne sommes pas 
en disaccord ici. En efTet, j'ai admis moi-m^me que, dans 
les taches, il se produit une absorption plus forte par la 
raison qu' /tant des cavites remplies de la "tnatihre de The spots 
r atmosphere transparente du Soleil, elks constituent une ^^^"^^ 
c0icAe plus pro/onde, ce qui implique bien que la base de abscrptiou. 
Fatmosphere soit au-dessous des taches, comme le veut 
M. Lockyer. Cela est encore plus clair dans Thypoth^se 
de Wilson et Herschel, d'apr^s laquelle la photosphere ne 
serait qu'un brouillard lumineux suspendu dans Tatmo- 
sph&re transparente, hypoth^se que jusqu'ici je juge la plus 
probable. Du reste, je ne sais pas quelles sont les affirma- 
tions auxquelles il fait ici allusion, surtout apr^s avoir 
d^lar^ que je ne combats pas sa theorie. 

** M. Lockyer conteste aussi ma conclusion, que la raie F Composite 
puisse 6tre compos^e, car il dit qu*il n*a pas observe les faits ^^^^^^ ^f 
que j'ai indiqu^s. Cela me parait inconcevable, car lui- 
mSrne, 4 la page 122, n** IX, il dit express^ment : lorsque la 
ligne brillante (F) et la ligne noire se trouvaient c6ti d c6ti, 
la dernihre itait toujours la moins refrangible. Or c*est la 
justement ce que j*ai observe et 6nonc^ de cette mani^re, 
que pr^s du bord la ligne brillante ne remplit pas toute la 
largeur de la ligne noire, mais en laisse une partie noire du 
c6t^ du rouge. Le fait est au fond le m^me, et si j'ai pu 
le constater, malgr^ sa d^licatesse, cela prouve bien que 
mon instrument n'est pas insuffisant. Si M. Lockyer a vu 
la raie F renvers^e, cela n*est pas surprenant, car elle doit 
se renverser comme la raie C, mais il sera bien difficile de 
constater le renversement total, car la lumi^re de la raie F 
est plus faible que celle de la raie C. 

" Du reste, il est difficile de juger des details de ces 
observations et de trouver la source des discordances entre 
les deux observateurs, sans connaitre k fond le syst^me 
d'observation employ^ par M. Lockyer. S'il ne grossit pas 
convenablement son image, il pourrait bien se faire que tous 



SOLAS PMVSICS. 

ct:s n^ouvements ct ces changements de refrangibililiS des 
raics, qu'il dit avoir observes, fiissent des illusions, j'aivu 
frt^quemment des mouvements semblables sc traduisant 
quclqucfois par unc duplication de la raie, mais je Icsai 
attribuus a I'agitation de notrc atmosphere, et a la chaleur 
soiaire agissaiit sur la fente, qui peut bien produire dcs 
tiOviations accidenteUes des rayons. Ordinal rem ent, ces 
phcnomd-ncs disparaissent en mettant bicn au foyer I'appa- 
rcil. Cela soit dit cependant sans revoqiicr en doute Ics 
■is.sertions de cet observateur eminent, mais sculcmcnt pour 
Ic mcttre en garde conlre unc cause d'illusion qui a d'autant 
plus d'influence que Ics images sont plus petitcs. 

"Pour n'ctre pas trop long, j'omettrai d'autres d^iU 
sccondaircs, ct jc tcrminerai par ce qu'il dit sur !e spectre 
obser\-e par moi dans I'intirieur des taches. Id il ne in'a 
pas utu aussi facile de saisir sa critique, car il dit que 'fai 
thrril lies pliaioini}u-s trh-cetnpHqiu's, aciontpagrtant ct spttin 
des teichcs', ce qui voudrait dire qu'en ri^alit'J Ics pW-nomencs 
.sont plus simples : ccpend,int il ajoute ' qu'avec trois prumeu 







views. 



OBSERVATORY WORK, 507 

To these remarks of Father Secchi I sent the following fifth 
reply ^ :- "*""'• 

** La partie de ma Communication qui a ^t^ imprimee 
dans les Comptes Rendus du 12 juillet, et qui a rapport aux 
observations du P. Secchi, avait pour objet, comme je Tai 
clairement indiqu^, Tdtablissement de certains points sur 
lesquels nos observations n'dtaient pas d'accord, afin que 
d*autres personnes puissent employer la nouvelle m^thode 
d'observation dans les meilleures conditions. 

"J'ai done ^te un peu surpris du ton de la reponse du 
P. Secchi, ton que je n*ai pas la moindre intention d*imiter 
dans le pr6sente Communication. 

Je trouve, dans la lettre du P. Secchi : ScccMs 

I** Qu'il tient encore k ' une couche donnant ;/;/ spectre 
continu^ couche qu'il considere comme la basede P atmosphere 
solaire et dans laquelle il pense que s'efftxtue le rcnversemcnt 
selon la th6orie de Kirchhoff;' 

"2® Qu'il a des doutes sur Timportance que j*attribue i 
r^largissement de la raie F i sa base, sur lequel le Dr. 
Frankland et moi avons fonde notre estime de la pression 
de la chromosphere ; 

" 3^ Qu'il 3. aussi des doutes sur les changements de 
longueur d*onde dans les raies de I'hydrogene que j'ai 
affirm^ 6tre continuellement visibles, sur le Soleil et en 
dehors de cet astre ; 

" ^ Qu'il soutient encore que la raie F est due i I'absorp- 
tion de quelqu'autre substance, en outre de Thydrogene. 

"Sur tous ces points, je m'en remets volontiers au juge- 
ment de Tavenir. Je puis cependant faire remarquer, quant 
k ce qui touche le premier de ces points, que quoique je 
ne voie rien qui ressemble k un spectre continu, je vois 
positivement des traces d'absorption r6duite dans la couche 
ext^rieure de la photosphere, et le Dr. Frankland et moi 
avons montr^ que Tabsorption augmente lorsque les couches 
inf(6rieures sont mises en action comme dans une tache. 
Le P. Secchi a dcrit r^cemment {Comptes Raidus, 1869, 2* 
* Comptes Rendus, vol. cit., p. 452, et seq,y August i6th, 1869. 




■ im boid ds disqae da 
qne CCS deux 

do bold' 
2 mc aOHtte qoe dest aac 

rdztmu 



i?-^, ; 






B, A^ lefMl nnftBoioe de h 
«■■% |e die cncoce {Cm^ 
P4l> :'£« encore port^ hhd 

>c<r;tiji. ::<:^=a££9i^«seBafiailtksiwe5pri]idpiles 

i .-.^ l:^;-:£SGi dk la. rusSKW czvmir fmr la 

r^ i^ i=./;-r^-3>r:T.' Ces txfnaaems OK faappent, coaunc 
'jiTZ =2.^ a-.-..-.elLec nnira«Bffi oa.akP.Secdiide»Ta dwiiir 
asi'iirtiraa oppori cfc Mail je dots loi reodre Li 
sd'z^rtrc qoil xrutd^ iMflgiuf qu'oa pounail 
r fea f ta t ra mo ^ 'gn d'expgrignces de 




OBSERVATORY WORK, 509 

est ^tabli que les premieres observations ont it6 faites les fifth 
21 et 28 Kvrier. ^^"^ 

''Quant k son assertion qu'il ^nonce ainsi : 'J'ai claire- TTuqties- 
meat vu et d^montre que seulement une ligne du magn^- **^jfi^ 
slum est renvers^e et que Tautre ligne brillante occupe 
Tespace interm^diaire des deux plus voisines/ je me 
ooosidire comme pleinement justifi^ en niant Texactitude 
de Tobservation, et je laisse cette question, comme les 
autreSy 4 la decision de Tavenir ; je ne m'aventurerais pas 
jusque-lii si une longue s^rie d'exp^riences, faites au 
Collie Royale de Chimie, n'avait pas ith compl^tement 
d'accord avec mes observations t^lescopiques qui ont d^j^ 
£t6 d^rites dans les Comptes Rendus. 

"LeP. Secchi me fait le reproche de mfiler la th^orie Value of 
avec les observations. J*avoue franchement qu'il en est ftf^ 
ainsL Je confesse qu'une remarque faite, il y a ddj^ 
quelque temps, par M. Faye est toujours pr^sente k mon 
esprit lorsque je me livre k des observations. Voici cette 
remarque : ' Une bonne th^orie est aussi n^cessaire qu'un 
bon telescope.' Sans une hypoth^se qui dirigeait mon 
travail, j'aurais bien certainement beaucoup moins interrog6 
le Soleil que je ne I'ai fait, et ce serait une naivetd de dire 
que, dans une recherche comme celle que nous poursuivons 
maintenant, il ne convient pas d'observer aveugl^ment ou 
au hasard. Par exemple, j'ai commence par m'appuyer sur 
la th^orie, gdndralement admise alors, que * I'absorption 
avait lieu en dehors de la photosphere/ ce qui est ^videm- 
ment Fid^e actuelle du P. Secchi, comme on le voit par 
Textrait d'une de ses derni^res Communications que j'ai 
d^ji cit6. Mais, en mettant k T^preuve cette th^orie de 
toutes les mani^res, j'ai reconnu qu'elle ^tait insoutenable, 
ct je croispouvoir dire que, si le P. Secchi avait fait comme 
moi, il y aurait eu moins de contradictions dans ce qu'il a 
cherch6 k ^tablir, et aussi qu'il aurait trouvd que cette 
thtorie n'est pas soutenable. Mais j'avoue que la remarque 
qu'il est trop t6t pour ^tablir une theorie, venant du P. 
Secchi, m'a un peu ^tonnd, car je trouve un grand nombre 



~= ^ = r=^^asaes|]n>prcs M^moires siirce 

T^i ir-. -^iv s K SoB^ j*£tabltss3is que, 'a ii 
Beocrtaine distance dell 
. iir.i^ =; pmmaaas tRm\-cr ud elargissemoit dt 
mt^ P. Secdii fait remarquer qu'i! 
~^ ?=i — "I ■■ ^ ««« conclusion. Cela peut 
^^^ js = -lT a'adad pn qoe la raie F s'cUigit pif 
is.i. Xi-:5 m RHBcqae s^ifie simplcment que, 
=~r!rLir;CT>^-: i:;j« SMDC ao Ucu d'etre epaissc, c'nt- 
s *_= T^ 5 'W i bil (OS jasqu'i la photosphere. 
i^TuT iitr.;rs is fAwe poor qne la pression dnieniif 
=i — i '^^^ itats: 'Cette structure des masses 

%,-:jss- i:;a^ _m: -^m e ef k in nisulte . ... da 

e^^i-rcs i.;i=- sdifaa.' Je demanderai alors oil de 
rcssmzanri 6e facfaoaiaqifa^Te (car les proeminences 
f-rr? i^ ^ rr^iii) cat etc ooosigni^es, et coaiment 
rrcs. a dks exisCail, pourraicnt nous semi 




OBSERVATORY WORK. 511 



" 5** Que rhydrog^ne est le plus l^ger de tous les gaz. fifth 

" Le P. Secchi regarde mes observations d'injections de ^^^^^' 



sodium, de magnesium, etc., dans la chromosphtrey comme 
une preuve de Texistence de la couche ou stratum k spectre 
continu qu'il place au-dessous de la chromosphere. Je lui 
demanderai comment cela est possible. Mes observations, en 
outre, d^montrent, je pense, que les vapeurs de sodium, de 
magnesium, etc, sont imm^diatement plac^es au-dessous de 
la chromosphere, et alors comment peuvent-elles donner 
au-dessus un spectre continu, si elles n'en donnent pas au- 
dessous } 

"Dans ma derniere Communication j'ai.dit qu'en em- influence 
ployant trois prismes, les ph^nom^nes des taches pouvaient ^^^^ 
6tre aussi compliqu^s que le P. Secchi les a d^crits, mais 
qu'avec les puissants pouvoirs dispersifs que j'emploie, cette 
complication disparait en tr^s-grande partie. A cette occa- 
sion le P. Secchi dit que *je cherche a mettre mes r^sultats 
en opposition avec les siens.* Je ferai remarquer que mon 
but c^tait justement le contraire, et je cite encore un des 
demiers M^moires du P. Secchi {Coviptes Rendus, 1869, 2*^. 
semestre, p. 166), d*une date plus r^cente que celui dans 
lequel il 6tablit que le spectre d*une tache est semblable i 
celui du limbe : * II n*y a pas production des raies fonda- 
mentales nouvelles, mais seulement un renforcement con- 
siderable des raies solaires connues comme d6ji existantes.' 
Sur ce point, je laisserai encore le P. Secchi se mettre 
d'accord avec lui-meme. Quant k ce qui touche spdciale- 
ment aux raies brillantes, visibles parfois dans les taches, 
que le P. Secchi regarde comme dues k la radiation du 
noyau gazeux int^rieur du Soleil (Comptcs Rendtis, 1869, 
I*', trimestre, p. 165), je puis dire seulement que je n*ai 
pas vu dans les taches de lignes brillantes qui ne fussent 
visibles en meme temps dans le spectre solaire ordinaire. 
II est vrai que, dans les taches, elles sont beaucoup mieux 
vues. 

" En terminant, je dois dire que la m^thode d'observation 
que j'cmploie, et dont je regrette de voir que le P. Secchi 



adndle da 

4e6:q poaocs ffouvettnre 

fciexi me imige 

rdatde 

sleplos dcns^qw 

t ^ pbs de 300 dcgns; 

: par tin aube 

e 60 depist et un pnsoK i 

.-^7-Q i^rcc^ : =^ poavow-; eofio. que je ne snis pas 
= :T:r:c; £iic:re :.: celfee tSapetsoo, (jni est pins qoe doatle 
ce Ciile q»i s=:j: :i£ le P. Seodli et fKSpin, dans qudqucs 
-ouTa. avMr 2. rD:k dopoAiaH a poovoir double de ceka 

liont ;i dtspcse aajfl 

Here 




Faiher Secdo's Ftjoada * : — 



P, 18691 



" Is \ieni de recevotr le C^w^U RgmJm An 16 aoOt dam 
ieq-e! ea: inscrc^ li rip o me de H. Lodcyer & ma denuere 
C vmmur.fcaLcn. Comme ce a'est pas moi qui ai ami- 




OBSERVATORY WORK. 513 

rintersit^ de la lumi^re pourraient bien y contribuer. fifth 

Cette seconde contradiction ne subsiste done pas plus ^^^^^- 
que la premiere. 



**3** Je n'ai jamais jetd de doute sur la v^racit^ de Observa- 
M. Lockyer comma il Tinsinue, p. 454, lig. 18. Cela a tou- f^^^- 
jours iii loin de ma pens^e. Mais j*ai dit que le ren- sium. * 
vcrsement observe par moi 6tait bien diff(6rent de celui 
qu'il a vu. J'ai vu (et je le maintiens, car Tobservation a 
dur^ deux heures et je ne me suis pas trompe) renvers^e 
seulement une raie du magnesium, et j'ai constat^ que 
rintervalle entre les deux autres ^tait devenu plus brillant. 
Cela explique, du reste, la double raie brillante vue dans 
r^lipse par M. Rayet, qui en a vu detix et non trois. Cela 
n'empeche pas la v^rit6 de Tassertion de M. Lockyer qui 
dit avoir vu les raies toute trois renversees. Mais cela 
serait une observation diff^rente et qu'on ne peut confondre, 
ni par le fait ni par la date, avec la mienne. 

"4** Je ne m'occuperai pas de cequi regarde les theories ; 
parce que si moi-mcme j*ai essaye quelque chose, dans ce 
sens, je crois que cela peut bien se faire tout en admettant 
une insuffisance d'un c6t6 et de Tautre. Mais pour ce qui 
regarde Tinexactitude qui ressortirait d'avoir affirmd que 
les masses suspend ues dans Tatmosph^re du Soleil sont le 
r^ultat des observations antirieures des eclipses, la chose 
est si bien connue, que je ne m*y arreterai pas. M. MathMs 
Mathieu le premier et apres lui un grand nombre d'obser- 
vateurs ont constat<5 les arcs roses outre les prodminenccs : 
or, ce n'est pas cela qui constitue ce que M. Lockyer a 
appel6 chromosphhe. Le nom sans doute appartient i M. 
Lockyer, mais la chose existait bien avant qu'ii eOt em- 
ploy^ cette d<l*nomination. 

" 5** M. Lockyer demande une demonstration de Texis- 
tence de ce milieu dans lequel peuvent nagcr ces masses 
d'hydrogtne. Je lui rcpondrai qu'il n'en faut pas chercher 
une ailleurs que dans le fait de leurs formes definies elles- 
mcmes, et que cette atmosphere est bien sensible dans les 
^Upses ^ une distance bien plus grande que n'atteigncnt 



obsenui- 
tions. 



SOLAK PHVStCS. 



les procmincnces et que nos photographies du Daurti 
nous out signals la forme elliplique de cette enveloppe plia 
relevee a I'eqiiateur qu'aux p61es du globe solaire. Ccttt 
atmosphere pcut bieii contenir de I'hydrogene plus froidel 
d'aiitres gaz rarefies, bien que Thydrogcoe soit le pliB 
l^gcr des gaz (ce qui est bien connu), mais que par I» 
diffusibilite propre a toutes les substances gazeuses il peut 
sc meler i d'autres d'un poids specifique plus grand. 

"6" Je ne comprendspasceque M. Lockyer dit relative- 
mciit aux vapeurs de sodium et de magnesium pbcts 
iiniiit'diatement au-dessons de la photosplibe (p. 456, lig, s). 
Je ne sais pas comment on peut admettre la possibilite de 
coiistatcr ce dcssous ; car la profondeur de la photosphere 
est insondablc pour nous. Autrefois j'avais cru moi-mfme 
que la profondeur des taches ^tait la mesure de lepaisseuf 
dc la couche photosph^rique ; mais cette throne aujourd'hui 
n'est plus soutenable, et en cela je n'ai pas de difficulte i 
admettre que je suU maintciiant en contradiction av« 
ce que j'ai avance autrefois. Jamais je nc rougirai de 




OBSERVATORY WORK. 515 

des inexactitudes provenant de ce que mes Communications fifth 
ont 6t6 trop abr^^es, je prierai TAcad^mie, dans une ^^^^^' 
prochaine Communication, d'accepter quelques pages de 
la traduction litt^rale de mon Journal, dans lequel mes 
observations sont enregistr^es avec les details n^cessaires 
pour ^viter toute interpretation erronee." 

Here I need scarcely say I let the discussion drop. It 
is consoling to know that all the points at issue have now 
been settled. 



L L 2 



SOLAK PHVS/CS. 



Sixth Paper. 

SfiCtn'SCi'pic Obscrvatians of the Stm. No. VI. — By J. 
N'yRMAX LOCKVER, F.R.S. From the Proceedings of 
the Royal Society, No. I20, 187a Received April ;?, 

TUF. weather lately has been fioc enough and the sun high 
L-dou^h during my available observation time to enable nic 
to resume work. 

The crop of new facts is not very large, not so large as 
it would have been had I been working with a strip of the 
sun, say fifty miles or a hundred miles wide, instead of 
uTic considcrcibiy over a thousand — indeed, nearer two 
thousand — in width; but in addition to the new fact? 
obtained, I liavc very largely strengthened my former 
ob^LTvation^, so that the many hours I have spent in 
watdiiiit: phenomena, now perfectly familiar to mc. have 




OBSERVATORY WORK, 517 



II. In the case of a bright prominence over a spot on the sixth 
disc, the C and F lines have been seen bright, while the _'*1!!5'!_- 
yellow line has been invisible. 

III. In a high-pressure injection of hydrogen the motion Motion in 
indicated by change of wave-length has been less in the f^^^^Q 
case of the yellow line than in the case of C and F. and v. 

IV. In a similar quiescent injection the pressure indi- Thepra- 
cated has been less. "~Xf "^ 

V. In one case the C line was seen long and unbroken. D3 broken 
while the yellow line was equally long, but broken. j^^vAo/e 

The circumstance that this line is so rarely seen dark 
upon the sun, makes me suspect a connection between 
it and the line at 5015 Angstrom, which is also a bright 
line, and often is seen bright in the chromosphere, and 
then higher than the sodium and magnesium lines, when 
they are visible at the same time : and the question 
arises, must we not attribute these lines to a substance 
which exists at a higher temperature than those mixed 
with it, and to one of very great levity ? for its absorption- 
line remains invisible, as a rule, in spot-spectra. 

I have been able to make a series of observations on 
the fine spot which was visible when I commenced them 
on the loth instant, not far from the centre of its path 
over the disc. At this time, the spot, as I judged by 
the almost entire absence of indications of general absorp- 
tion in the penumbral regions, was shallow, and this has 
happened to many of the spots seen lately. A few hours* 
observation showed that it was getting deeper apparently, 
and that the umbrae were enlarging and increasing in 
number, as if a general downsinking were taking place ; but 
clouds came over, and the observations were interrupted. 

By the next day (April 11) the spot had certainly T?/-/,/// 
developed, and now there was a magnificently bright promi- ^^^"^^^""^^ 
nence, completely over the darkest mass of umbra, the umbra. 
prominence being fed from the penumbra or very close to 
it, a fact indicated by greater brilliancy than in the bright 
C and F lines. 



still per ' 

of im- 

afeervards I iras 

I dttak, dcpcdi!- 

cetaaUy oat it 

towards 

to roe 

av tke bbc3c C line 

C bar (on wtidi 

■• wdB visSiIe as to the 

tkc^nrptMO-tine for a 

B of tbe Use towanb 
I flf BKitioa ] got In 
-:=r :^ - ':3:cs Ac io^l^ aad daric Goes were coin- 





OBSERVATORY WORK. 519 



spots. These are the " radiance " and dull prominences sixth 
shown in the American photographs. ^^^^^ 

I now return to my observations of the spot On the 
l6th the last of the many umbrae was close to the limb, 
and the most violent action was indicated occasionally. 
I was working with the C line, and certainly never saw 
such rapid changes of wave-length before. The motion 
was chiefly horizontal, or nearly so, and this was probably 
the reason why, in spite of the great action, the promi- 
nences, three or four of which were shot out, never rose 
very high. 

I append some drawings made, at my request, by an Mr. Hoii- 
artist, Mr. Holiday, who happened to be with me, and ^^y* 
who had never seen my instrument or the solar spectrum 
widely dispersed before. I attach great importance to 
them, as they are the untrained observations of a keen 
judge of form. 

The appearances were at times extraordinary and new 
to me. The hydrogen shot out rapidly, scintillating as it 
went, and suddenly here and there the bright line, broad 
and badly defined, would be pierced, as it were, by a 
line of intensely brilliant light parallel to the length of the 
spectrum, and at times the whole prominence spectrum 
was built up of bright lines so arranged, indicating that 
the prominence itself was built up of single discharges^ shot 
out from the region near the limb with a velocity some- 
times amounting to 100 miles a second. After this had 
gone on for a time, the prominence mounted, and the 
cyclonic motion became evident ; for away from the sun, 
as shown in my sketch, the separate masses were travelling 
away from the eye ; then gradually a background of less 
luminous hydrogen was formed, moving with various velo- 
cities, and on this background the separate "bombs" Bombs! 
appeared (I was working with a vertical spectrum) like 
exquisitely jewelled ear-rings. 

It soon became evident that the region of the chromo- 
sphere just behind that in which the prominence arose. 



SOLAS /"WKsacs: 

Y'rLr'ZK.~.7^ zr::.-^ 6tt£t with a velocity sorocthiag like t^tnly 
TT-.lt^ s se-:^:- i. the back-ni^ beiitg so local that with the 
sriuill inu^ I am tin fortunately compelled to use, bc-th 
the itioviBg and rigid portions were included in the thkl;- 
-es> ■ i'Jm sl.t. I saw the two absorption-lines mcrlap 

Tiiiiic i5!i.v;r\-3ti':>as were of great importance to me ; im 
■ii-.i Tif'i 3«'on enabled me to put together several phctio- 
TSiiia I na^ perfectly familiar with separately, and s« 
nr^r cor.nected meaning. 

They may bo sumioarized as follows, and it will be wtn 
that they teach ;;s much oonceming the nature of prosii- 
nences, Whe:i the air is perfectly tranquil in the nei^- 
bourhood of a large spot, or, indeed generally in any part 
of the disc, we sec absorption -lines running along the 
whole length of the spectrum, crossing the Fraunhofer 
lines, and tliey vary in depth of sbade and breadth accord- 
ing as we have pore, corrugation, or spot under the cont- 
^ponding part of the slit — a pore, in fact, is a spot. Here 
and there, where the spectrum is brightest (where a briijht 




OBSERVATORY WORK. 521 

observation. In such cases the motion is cyclonic in the sixth 
majority of cases, and generally very rapid, and — another ^^^^^' 
feature of a solar storm — the photospheric vapours are 
torn up with the intensely bright hydrogen, the number of 
bright lines visible determining the depth from which the 
vapours are torn^ and varying almost directly with the 
amount of motion indicated. 

Here, then, we have, I think, the chain that connects 
the prominences with the brighter points of the facute.^ 

These lozenge-shaped appearances, which were observed ConmcHon 
close to the spot on the i6th, were accompanied by the ^j^^^j^ 
** throbs " of the eruption, to which I have before referred, jfacu/a. 
While Mr. Holiday was with me — a space of two hours — 
there were two outbursts, separated by a state of almost 
rest, and each outburst consisting of a series of discharges, 
as I have shown. I subsequently witnessed a third out- 
burst. The phenomena observed in all three were the 
same in kind. 

On this day I was so anxious to. watch the various 
motion-forms of the hydrogen-lines, that I did not use the 
tangential slit. This I did the next day (the 17th of 
April) in the same region, when similar eruptions were 
visible, though the spot was no longer visible. 

Judge of my surprise and delight, when upon sweeping Reversal of 
along the spectrum, I found HUNDREDS of the Fraunhofer ^J^lifl",] 
lines beautifully bright at the base of the prominence ! ! ! at the base 

The complication of the chromosphere spectrum was ^{Jj^^j^g' 
gfreatest in the regions more refrangible than C, from E to 
long past b and near F, and high-pressure iron vapour was 
one of the chief causes of the phenomenon. 

I have before stated to the Royal Society that I have 
seen the chromosphere full of lines ; but the fulness then 
was as emptiness compared with the observation to which 
I now refer. 

A more convincing proof of the theory of the solar con- 
stitution, put forward by Dr. Frankland and myself, could 

* See Note M. 



SOLAR PHYSICS. 

scarcely have been fomisiied. This tAsava^m not only 
endorses all vny f^rx^er work m this dirocttoa. bat it tcndi 
toshrrw-the -'t..-. 'vness <rf the rcgioQ in n-hich mxay n\ 
the more im: r -: solar phenomena Uke place, as well 
as it'; exact '.■■':.. ■.;■■, 

The appearance; of the F liee. with a tangential slit al 
the base i>r the promiaeDGc, iochided two of the lozenge 
shaped, brilliant spots to which I have bcTore reTencd^ 
they i.vere more elongated than usual — an effect of pmsuic, 
I hold ; greater pressure and therefore greater compKaiion 
of the chromosphere spectram ; this ccmplication is il- 
mo5t impossible of observation on the disc 

It is noteworthy that, in another prominence, on the 
~ame side of the sun, although the action was great, the 
Lrupted materials were simple, tV. only sodium and mag- 
nesium, and that a nicxleratc alteration of wave-length in 
thijse vapours was ob%'ious. 

Resides thtic obser\-ations on the ly-th. I also availed 
myself of the piireness of the air to telescopically examin 




■■in 



■n 



C/ 



flijfl 






*■'*■ i4f.—"itolicii Furmi." and "I.fllenget." (S« sent page (or dnci^VtVno^ 



1 






X fiMb — tv^c''' t'JiC cnssn^ bint 



V ccnnes (trf F iataii 



j. 13. 1:1. II. : 5 "y— ' ' i-|- ——**-■ — i-jc_— J— -J 
ij. Akaoigsii seem « the sm it"" 



« 




U.— LABORATORY WORK. 
First Paper. 

Preliminary note of Researches on Gaseous Spectra in rela- 
tion to the Physical Constitution of t/te Sun. By 
Edward Frankland, F.R.S. and J. Norman Lock- 
YER, F.R.A.S. Received February ii, 1869.^ 

I. For some time past we have been engaged in a careful first 
examination of the spectra of several gases and vapours p^ p^R- 
under varying conditions of pressure and temperature, 
with a view to throw light upon the discoveries recently 
made bearing upon the physical constitution of the sun. 

Although the investigations are by no means yet com- 
pleted, we consider it desirable to lay at once before the 
Royal Society several broad conclusions at which we have 
already arrived. 

It will be recollected that one of us, in a recent commu- Our stari- 
nication to the Royal Society, pointed out the following Z*^"'^- 
facts : — 

1. That there is a continuous envelope round the sun, 
and that in the spectrum of this envelope, which has been 
named for accuracy of description the " chromosphere," 
the hydrogen line in the green corresponding with Fraun- 
hofer's line F, takes the form of an arrowhead, and widens 
from the upper to the lower surface of the chromosphere. 

2. That ordinarily in a prominence the F line is nearly 
of the same thickness as the C line. 

' Proc, A'.S.j vol. xvii. p. 288, 



SOLAS PHYSICS. 

3. That sometimes in a promiuence the F line is ex- 
ceedingly brilliant, and widens out so as to present > 
bulbous appearance above the chromosphere. 

4. That the F line in the chromosphere, and also thcC 
line, extend on to the spectrum of tlie subjacent regioos, 
and re-reverse the Fraunhofer lines. 

5. niat there is a line near JD visible in the spectnimof 
the chromosphere to whidi there is no cofresponding Fraun- 
hofer line. 

G. That there are niany bright lines visible in the 
ordinary solar spectrum near the sun's edge, 

7. That a new line sometimes makes its appearance in 
the chromosphere. 

II. It became obviously, then, of primary importance — 

1 . To study the hydrogen spectrum very carefully under 
varying conditions, with the view of detecting whether or 
iiot there existed a line in the orange, and 

2. To determine the cause to which the thickening of 
the V line is due. 

have altogether failed to detect any line Jn iht 




LABORATORY WORK, $27 



nence, in which the red and green lines are nearly of first 

equal width, and in the chromosphere, through which the _^ !_ 

green line gradually expands as the sun is approached.^ 

With regard to the higher prominences, we have ample Ijmnessot 
evidence that the gaseous medium of which they are ^tXechro^ 
composed exists in a condition of excessive tenuity, and mosphete, 
that at the lower surface of the chromosphere itself the 
pressure is very far below the pressure of the earth's 
atmosphere. 

The bulbous appearance of the F line before referred to 
may be taken to indicate violent convective currents or 
local generations of heat, the condition of the chromo- 
sphere being doubtless one of the most intense action. 

IV. We will now return for one moment to the hydrogen Hydrogen 
spectrum. We have already stated that certain proposed ^P^^^*^- 
experiments have not been carried out. We have post- 
poned them in consequence of a further consideration of 

the fact that the bright line near D has apparently no repre- 
sentative among the Fraunhofer lines. This fact implies 
that, assuming the line to be a hydrogen line, the selective 
absorption of the chromosphere is insufficient to reverse 
the spectrum. It is to be remembered that the stratum of 
incandescent gas which is pierced by the line of sight along 
the sun's limb, the radiation from which stratum gives us 
the spectrum of the chromosphere, is very great compared 
with the radial thickness of the chromosphere itself; it 
would amount to something under 200,000 miles close to 
the limb. 

Although there is another possible explanation of the 
non-reversal of the D line, we reserve our remarks on the 
subject (with which the visibility of the prominences on 
the sun's disc is connected) until further experiments and 
observations have been made. 

V. We believe that the determination of the above- 
mentioned facts leads us necessarily to several important 
modifications of the received theory of the physical con- 

^ Will not this enable us ultimately to determine the temperature ? 




■ theory we owe 
-vm: xsstA ft *pBB li>B camninatioo of the 
■r *• Ah ^podKsis, the photo- 
■i or B^wi and it is surrouiuicd 
ncd «f ^scs *iid the vapours of 
K m tfec pkofeospbcre. 
: imn ]>:<V7i^r nt^al of tkis cm^ioaad atmospbcR, 
Tiiri ~r^ is ■eMi)r «- at an events main^ tbc 
-in ;t "17-^ :^?ta ; pt is mat, bmrever, oomposed 
^nr-Ij It iy-ir-ig^eB alsae; aad tias potat is eog^ng 
zt;:^^uL irr=ri:c ;) sad Ac t fuitj of tlus iocandescent 
f^iT-::^ :s- sz:h. tkadt ft is cUiuiMJy improbable dot 
r.'£r^cerx:v>; ' in mihiiL aKJhaa the cororu has been 
"i^' r: TCL' Try . fcs owtadt it, — a view strengthcnnj 
;^ *-_~ tiii i^ cteaaniiKfic bcigfat lines present no 
i.-Tr.rs :c iz^-rpboa. lad that its pSiysica] conditioiB 

-r-^i-: - -^c (A i ift O Bff t gr e ItseM; so far from being 
^oe Off a fiqaid ocean, that it is clouJy 




LABORATORY WORK. 529 
— ^ 

conclusion strengthened by the consideration that first 
otherwise the newly-discovered bright lines in the solar ^^^^^' 
qpectrum itself should be themselves reversed on Kirch- 
hoflTs theory? This, however, is not the case. We do 
not forget that the selective radiation of the chromosphere 
does not necessarily indicate the whole of its possible 
■elective absorption ; but our experiments lead us to believe 
that; were any considerable quantity of metallic vapours 
present, their bright spectra would not be entirely invisi- 
ble in all strata of the chromosphere. 



M M 



SOLAR PfiYSICS. 



Second Paper, 



Researches on Caseous Spectra in relation to tkt Physical 
Coiistitiilion of the Sua, Stars, and Nebula. — Second 
Note. By E. Fkankland, F.R.S.,and J. N. LocKVEK,— 
From the Proceedings of the Royal Society, No. li:, 

1S69.' 
\Vl beg to lay before the Royal Society some further 
results of the researches in which we are engaged. 

I. The Fraunhofer line in the solar spectrum, named A 
by Angstrom, which is due to the absorption of hydrogen, 
is not vi.sibic in the tubes we employ with low battery and 
Lcydcn-jar power ; it may be looked upon therefore as in 
indication of relatively high temperature. As the line in 
question has been reversed by one of us in the spectrum 
of tlie chromosphere, it follows that the chromgsphcie, 
when cool enough to absorb, is Still of a relatively high 
temperature. 




LABORATORY WORK. 531 

spectra referred to in II. and III. were reduced to the two second 
bright lines. ^''^^^' 

VI. By reducing the temperature all spectroscopic 
evidence of the nitrogen vanished ; and by increasing it, 
many new nitrogen lines make their appearance, the 
hydrogen line always remaining visible. 

The bearing of these latter observations on those made Bearing on 
on the nebulae by Mr. Huggins, Father Secchi, and Lord ^'^^^^^^ 
Rosse is at once obvious. The visibility of a single line 
of nitrogen has been taken by Mr. Huggins to indicate 
possibly, first, "a form of matter more elementary than 
nitrogen, and which our anal3'sis has not yet enabled us to 
detect,"^ and then secondly, "a power of extinction exist- 
ing in cosmical space." ^ 

Our experiments on the gases themselves show not 
only that such assumptions are unnecessary, but that 
apectrum analysis here presents us with a means of largely 
increasing our knowledge of the physical constitution of 
^hese heavenly bodies. 

Already we can gather that the temperature of the 
nebulae is lower than that of our sun, and that their 
tenuity is excessive ; it is also a question whether the 
crontinuous spectrum observed in some cases may not be 
due to gaseous compression. 

* Phil, Trans, 1864, p. 444- ' Ibid. 1868, p. 544. 



MM 2 




i^ffrufuuium oj ine ohr, ^mrs, ana ivemu. 

By E. Frankland, F.R.S., and J. NoR 
F. R.S. — From the Proceedings of the 
No. 115. 1869.' 

THimi I- It lias been pointed out by one of us t 
''*^^"- of magnesium, iron, &c. are sometimes ir 

sun's chromosphere, and are then rendei 

their bright spectral lines.' 

II. It has also been shown (i) that these 

most part, attain only a very low elevatior 

sphere, and (2) that on rare occasions 

vapour is observed like a cloud separ 

photosphere. 
7hetua^- III. It was further established on the 
"liHis'of 1869, and a drawing was sent to the Rt 
vnrqual dicating, that when the magnesium vapour 
'^ ' the spectral lines do not all attain the same 
Thus of the b lines, b^ and i' are of neai 

but i5* is much shorter. 

IV. It has since been discovered that 
lines observed by Angstrom, only a very f* 
in the spectrum of the chromosphere wher 
injected into it. 

V. Our experiments on hydrogen and r 
us at once to connect these phenomena, al 



-.RSED IN THE SUN AND VISIBLE 

To fate p. 531. 
C B 

fl(3 q 4 a » m •Y «|* 6|6 





■ii -M zz^ 






LA BORA TOR Y WORK. 5 33 

simpler where the density and temperature were less, to third 
account at once for the reduction in the number of lines ^^^^^ 
visible in those regions where, on our theory, the pressure 
and temperature of the absorbing vapours of the sun are 
at their minimum. 

VI. It became important, therefore, to test the truth of 
this assumption by some laboratory experiments, the 
preliminary results of which we beg to communicate in 
this Note, reserving details, and an account of the further 
experiments we have already commenced, for another 
paper under the above title. 

We took the spark in air between two magnesium poles, 
so separated that the magnesium spectrum did not extend 
from pole to pole, but was visible only for a little distance, 
indicated by the atmosphere of magnesium vapour round 
each pole. 

We then carefully examined the disappearance of the b behaves 
b lines, and found that tlicy behaved exactly as they do on the ^^j^'/^j^y 
sun. Of the three lines the most refrangible was the doesinthe^ 
shortest ; and shorter than this were other lines, which one "***' 
of lis has not yet detected in the spectrum of the chromosphere. 

This preliminary experiment, therefore, quite justified 
our assumption, and must be regarded as strengthening the 
theory on which the assumption was based — namely, that 
the bulk of the absorption takes place in the photosphere, 
and that it and the chromosphere form the true atmosphere 
of the sun. In fact, had the experiment been made in 
hydrogen instead of in air, the phenomena indicated by 
the telescope would have been almost perfectly reproduced ; 
for each increase in the temperature of the spark caused 
the magne.sium vapour to extend further from the pole, and 
where the lines disappeared a band was observed surmount- 
ing them, which is possibly connected with one which at 
times is observed in the spectrum of the chromosphere 
itself when the magnesium lines are not visible. 



534 



SOLAR PHYSICS. 



Fourth Paper. 



FOURTH 
PAPER. 



Exptri' 

menfs with 

sjdium 

va/our. 



Researches in Spectrum Analysis in connection with tkt 
Spectrum of the Sun. By J. NORMAN LoCKYER, F.ILS 
Received November 6, — read December 12, 1872.' 

The researches, of which an account is g^ven in the present 
communication, have been undertaken in continuation cf 
those carried on by Dr. Frankland and myself at tk 
Royal College of Chemistry, from which v%'e arrived at the 
conclusion that the thickening of spectral lines was due 
to pressure, and not to temperature per se.'^ In our joint 
communications we pointed out that this held good for 
hydrogen in Geissler's tubes and for magnesium vapour,' 
when the spark was taken in air, by means of a method 
which enabled us to spectroscopically examine its various 
portions. 

The magnesium experiment was important not only so 
far as the decrease of thickness of lines with decrease of 
density was concerned, but because it showed that one of 
the well-known triple lines in the spectrum of magncsiuni 
absolutely vanished altogether from the spectrum at sorcc 
distance from the source of the supply of the vapour- 
that is, the pole of metallic magnesium. This result vc 
also obtained, as stated in our note, when we obscnei 
the spectrum of the spark between two magnesium poles 
enclosed in a Geissler s tube in an atmosphere of hydrogen 
in which the pressure of the gas was gradually reduced. 

In some experiments with sodium vapour which were 
not referred to in the papers in question,* Dr. Frankland 
and myself observed the same phenomena. The cxpcn- 
ments were conducted as follows : — 

* Philosophical Transactions ^ 1873. 

■ Pro:eedings of the Royal Society y voL xvii. p. 289, ante^ p. 526. 
J Ibid. vol. xviii. p. 79, ante^ p. 532. 

^ This experiment was first exhibited at a Lecture given by use ^ 
the Royal Institution in May, 1869. 



LABORATORY WORK. 535 



( I .) I nto a piece of hard glass combustion-tube, thoroughly focrth 
cleaned and closed at one end, a few pieces of metallic ^^"'' 
sodium, clean and as free as possible from naphtha, were 
introduced. The end of the tube was then drawn out and MaJksJtf 
connected with a Sprengel pump and exhausted as rapidly *^*2S^ 
as possible. Hydrogen was then admitted, and the tube 
re-exhausted, and, when the pressure was again reduced to 
a few millimetres, carefully sealed up. The tube thus 
prepared was placed between the slit plate of a spectro- 
scope and a source of light giving a continuous spectrum. 

Generally, unless the atmosphere of the laboratory was 
very still and free from dust, the two bright D lines could 
be distinctly seen on the background of the bright con- 
tinuous spectrum. 

The tube containing the sodium was then heated with 
a Bunsen flame, and the spectrum carefully watched. 
Soon after the application of the heat, a dark line, thin 
and delicate as a spider's thread, was obser\'ed to be slowly 
creeping down each of the bright sodium lines and exactly 
occupying the centre of each. Next, this thin black line 
i¥as observed to thicken at the top where the spectrum of 
the lower denser vapours was obser\'ed, and to advance 
downwards along the D line, until arriving at the bottom 7VD 
they both became black throughout ; and if now the heat ^^^^ST 
ivas still applied, thus increasing the density of the various 
layers of the sodium vapour, the lines began to broaden 
until, in spite of considerable dispersion, the two lines 
blended into one. The source of heat being now removed, 
the same changes occurred in inverse order; the broad 
band split into two lines, gradually the black thread alone 
was left, and finally that vanished, and the two bright 
lines were restored. 

(2.) This experiment was then varied in the following 
way: — Some pieces of metallic sodium were introduced 
into a test-tube, and a long glass tube conveying coal-gas 
passed to the bottom, an exit for the gas being also pro- 
vided at the top. The sodium was now heated and the 



S36 



SOLAR PHYSICS. 



FOURTH 
PAPER. 

Th€ black 
lifus thin 

OMiwfun 
the vapour 

' is diitUjti. 



flow of coal-gas stopped. In a short time the reversal of 
the D lines was complete. The gas was now admitted, 
and a small quantity only had passed when the black lines 
were reduced to threads. 

In my former communications to the Royal Society 1 
have pointed out the extreme importance of these facts it 
connection with solar and stellar physics. In observing 
the sun by the new method, we get various Fraunhofcr 
lines thickened in the spots and thinned in the chromo- 
sphere and prominences; and in these latter, in some 
instances, notably in the case of F, we find the lines 
gradually widening as they approach the limb of the sun. 

While this may be remarked as a solar demonstrari.n 
of the correctness of the conclusion at which Dr. Frankland 
and myself had arrived, it is to be noted that britjht !:n; 
prominences may occasionally be seen on the sun's <ii^- 
over or near spots in the spectrum of which the same line> 
are thick, while this phenomenon could not exist if the 
thickening of the lines were due to temperature alone. 



A section 

of (he 
spark is 
observed. 



Method Employed.^ 

The method of observing spectra to which I have 
already referred, and which has been adopted in the ^K■^:'^ 
of which I now propose to give an account, consists ia 
throwing an image of the spark on the slit of a spectr- 
scope in the laboratory experiments in exactly the sane 
manner in which I proposed, in 1866, that an image of the 
sun should be thrown on the slit in order to spectre- 
scopically examine minute portions of the sun and hb 
surrounding atmosphere. 

It is obvious that in this method the image of the C\\ 
will be associated in the spectroscope with an image of i 
section of the spark ; and that if from any cause there be 

1 This method was first exhibited at a lecture at the Royal Insdti- 
tion April 2nd, 1870. The same method has more recently beet 
employed with great success by M. Salet in a research on the specin 
of the Metalloids. 



LABORATORY WORK. 



S37 



various shells of vapour surrounding each pole, which 
shells give different spectra, then these spectra will be 
sorted out so that their variations may be traced from pole 
to pole. 

The arrangements adopted will be easily gathered from 
the annexed woodcut (Fig. 150) and the accompanying 



FOURTH 
PAPER. 




Fic. 15a — A, collimator ; B, ob<>crving>telescope ; C| %park ; d, len.i. 



description. It is scarcely necessary to add that an im- 
portant condition of this new method is that the object- 
glass of the collimator should be filled with light, and also 
that no light should be wasted. So long as these conditions 
obtain, conjugate foci and different lenses may be employed 



538 



SOLAR PHYSICS. 



FOURTH 
PAPER. 

Spectro* 
scope. 



Coil, 



PhofO' 
graphic 
arrant^C' 

ments. 



and the size of the image varied at pleasure, and stili the 
brightness of the spectrum will be sufficient. 

The instruments with which the observations have been 
made are as follows : — A large spectroscope, a sister in- 
strument to that used by Bunsen and Kirchhoff in thdr 
celebrated researches, and made by the same maker, 
Steinheil, of Munich.^ It is furnished with four prisms of 
flint glass. Three are of an angle of 45'' and one of 6o\ 
The general arrangements of the instruments are described 
by Kirchhoff in his memoir. 

In front of the slit plate is placed a lens throwing on 
the slit the image of the spark. 

A coil, made by Apps, and giving a 4- inch spark. 

A large Leyden jar has also been occasionally used as 
a condenser on the secondary wire. 

Beneath the observing-telescope is placed a commutator, 
by which the current is controlled by the observer without 
changing his position. 

The window of my laboratory looks due south, and the 
collimator is placed in the same direction ; and when it 
became necessary to have the solar spectrum in the field, 
the light reflected from a hcliostat placed outside the 
laboratory in direct prolongation of the line of collimatioa 
was thrown on to the lens and thus on to the slit, where 
the size and intensity of the images could be varied at 
pleasure by altering the position of the lens. 

When it was required to photograph a spectrum, the 
ordinary observing-telescope of the spectroscope was dis- 
mounted, and its place supplied by a telescope of 3 J inches 
aperture and 49 in. focus. This was suppK)rtcd on the 
cast-iron table of the spectroscope at one end, and at the 
other on a stand. The eyepiece and its mounting ^eie 
removed, and against the end of the tube, thus left free 
a small camera-box holding a plate 4J in. by l\ ia 

' This spectroscope has been temporarily placed at my disposal b» 
Professor Guthrie, of the Royal School of Mines, to whom my bo: 
thanks are due. 



LABORATORY WORK. 539 

was placed, and the photograph taken in the usual manner, fourth 
the focus being obtained partly by careful observation ^^^^^' 
with powerful magnifiers, and partly by trial-plates. 

From the time of Wheatstone's first experiments, when 
in 1835 he stated that if the poles consisted of two dif- 
ferent metals the spectrum contained the lines of both 
metals, down to the researches of Stokes, Miilcr and 
Robinson in 1862, there is no reference, so far as I can 
find, to any localization of light in any portion of the 
ireadth of the spectrum. In the case of the spark taken 
between two poles, e.g., in air, the spectrum is gene- 
rally one in which the lines of the two vapours and of 
air are blended together, all the lines running across the 
field. 

But under certain conditions this is not so. Thus SUckeis 
Stokes,^ who used the spark itself instead of a slit, re- ^l^^' 
marked that the metallic lines are "distinguished from air- 
lines by being formed only at an almost insensible distance 
from the tips of the electrodes, whereas air-lines would 
extend right across." 

Miller,* who used a slit and a spark close to it, referring MilUr. 
to his photographs of electric spectra, remarks : — ** The 
marginal extremities of the metallic lines leave a stronger 
image than their central portions," and the extremities of 
these interrupted lines he terms " dots." 

On the same subject Robinson * writes : — *' At that boun- Robinson. 
dary of the spectrum which corresponds to the negative 
electrode (and in a much less degree at the positive ex- 
tremity) intense lines are seen, . . . which, however, are 
short." 

Thal^n (though he also did not adopt the method used Thalin. 
by Dr. Frankland and myself in and since 1869) observed 
this localization to a certain extent, doubtless on account 
of the long collimator which he employed. 

■ Philosophical Transactions, vol. clii. 1862, p. 603. 
» Op. cit, p. 877. • Op. cit., p. 947. 




I 



soiAK rtrvsics. 



Hi II Mill I* — ^"11 ya ansa dcs rates brilUntcs qa'ot 
, a'ab^nc ^mt daas dcs cas esueptionncls, commc, pv 
^ "™I*^ qmad la qaantite de la substance, soumbe i 
rcxpcfi^Ke. est tifaabwwfante ou quand riacandcsceocc 
^f^^ uu-iiR- Ces raies qui se prcsenteat ordinaire- 
beM aax bofdi da spectre sous la forme de points 
d^^f^ttr mitmt qoand les autrcs raics du metal fonnat 
d^ fipaa OMCiBaes ca travers du spectre, oat etc repre- 
maeeE MT la y**-''**^ par dcs ILgaes trcs-courtts." 

Bdfaie I pfDoeed furtber I \x% to refer to the two an- 
WKxeA i wj d L i ts (Fis& 151. 152). copied from irfiolOf^nphs 
■f a pstt of the ^icctrum observed when the )ar-!9aftE 
pOBcs (I) be f ec J i tbe pedes of sate and radmimm, tad {i. 




I je tKc en .-^miam and /ivi/. and the im^gi: i 
tte sEt It will be seen that in the case of these metalTK 
\tpomn (and it is tnie of all others that I have yet observtd). 
the lT f!T . as in the before-mentioned case of the triple line 
{*) of m^BCsium, are of unequal length, and that in the oc* 
OMchod of observation the lines in the spectra of the twn 
mctallk: vapours and of the air are separated in the 
dearest aad most cooveoient manner, the air-lines goii« 
cwLt across, and the lines of the metallic vapours extend- 
ing to greater or less distances from each pole, and in 
some cases (i^•. of the longest lines) overUpping. 

' *■ VAnoire sw U determination des longueurs d'onde its M^ 
„rful^<pc^- p. ,^ (riW-1 in the Nova Act, Regi* S«:iei»ti.; 
'" ill, scr. iiL vol. "■ I'pMla. >«»■ 



I 




LABORATORY WORK. 



^ 



With this communication are maps (Plates XI. XII. fodkth 
Kill-) of the spectra of the following elements made on ''*^"' , 
s method, the jar being used : — Na, Li, Mg, Al, Mn, Co, Eltmaai 
fi, Zn, Sr, Cd, Sn, Sb, Ba, and Pb, The lines were laid «"/^. 
[own from Thalen's maps, given in the memoir quoted 
and on the same scale — namely, 2 centimetres to 
Lch j^ij miUim. of wave-length. The spectra were then 
"efully and repeatedly observed, and the comparative 
■'lengths of the lines estimated and laid down over their 
respective wave-lengths. 




PI At the same time that these spectra have been mapped 
^th the spark taken in air. many of them have also been 
observed when their metals were enclosed in tubes and 
subjected to a continually decreasing pressure, as in the 
case of the before-mentioned experiment with magnesium. 
In all tJuse experiments it was found that the longest lines 
invariably remained visible longest. 

In the case of zinc the effect of these circumstances was 
very marked, and they may be given as a sample of the 
phenomena generally observed. When the pressure-gauge 
connected with the Sprcngel pump stood at from 35 to 40 
millimetres, the spectrum at the part observed was normal, 



BtAavinur 
0/ Ihe 



J 



SOLAX fKrSKS, 

=r7C liiz ;:ie - r-a &es «9S< and -491 1 ' (both of ithid^ 
k:c=i :3ir 5pc:r-. - a aha mcd ^Hfer the normal prcssoi^ 
s-^ .uie: >-£:i -. ^^-ni^ soc ansidaably rcduod ■' 
»i3=L Ci-ii; T.r» bdag •Carted tfacK litKs ra[>tdly(!6: 
:rT.a^i=; jr >^z:jr--- » *a ifcc Gae at 4679 : 4810 and 471I, 
a=3ir i- imvi- I -— -«1 bJ : at Ikst the two at 49J4 arlI 4911 
iTiin^eiDc:. is . l .(6^ aad ap pe al ed only at iatcn-aU « 
fC'.-G :n li; ;. ^. tbe&vB 4S10 aod 4721 remaining iitlfc| 
.laiia:^-! 3; j:^^-^ ikaiq^ ««di in bhUianc>-. This c)q)fr 
-;tMar »ri:? riritiirdfaBr time^iad 00 each occasion ilit' 
r^^^ »^5 JD^c t3 be alaoet at tlK same point, vu,— 
li^ xs5fT^i':-:^«At>thefiBe549£4afid 4911 

»g-= ~-± :ac fca i E" stood at .... 30 mm. 

rrc o; jg , 

r^ ^r - . . 29 ,. 

jix d;- . jr „ 

A rae ^:^ u i--..i^=aecre? »^s Kiftidcnt to restore the 




LABORATORY H^OR/C. 



cases that the difference between the spectrum of the 
chloride and the spectrum of the metal was : — T/iat under 
tlu same spark conditions the short lines were obliterated, 
while the air- lines remained unchanged in thickness. 

Changing the spark-conditions by throwing the jar out of 
the circuit, this change was shown in its strongest form, the 
final results being that only the very longest lines in the 
spectrum of the metal remained. 

The following are the details of the experiments made 
under these conditions : — 

Method of Obscrx-ation. — Some pieces of stout alumi- 
nium wire, lO miUims. long and 

3 millims. in diameter, were 

taken ; one end was flattened 

for about one-third of the length 

for the purpose of inserting it in 

the spark-holder, and the other 

was drilled down in the direction 

of the axis for from 2 to 3 

millims., and thus formed into a 

small conical cup ; a very fine hole 

was then drilled through the side 

of this cup at the bottom, and 

the flattened end carefully split. 

Through the lateral hole a piece 

of platinum wire, 05 millim. in 

diameter, was passed, and one end 

brought round through the split 

end of the aluminium, while the 

other was brought up the centre of 

the cup. The split was now closed 

by strong pressure in a vice, and the 

ends of the platinum wire cut ofl". 

The whole now presented the 

appearance of a small candle, the 

platinum wire representing the ""'"""""'-'"■ 

wick: the accompanying figures (Figs. 153 — 154) will render 








h 



ac> Round this wkk the 

e u iiL^ -lewder mat t i ^ tfl y nunmcd down. [K 

=m. -nuj-cc Ae vic^ was used Tor the cxamiio- 

i :>= ~:'s ~r^ «( ^BbSc banam, stroQtium and 

■cfcd into it.] One o( that 

I Ike knrcr pole in the spark> 



I Tie =^-dfc 

Q 



/?^ 



n^ 



LABORATORY WORK. 545 



is observed, the red line, 6705 2, is seen right across the fourth 
spectrum; the orange, 61020, is faintly visible for about ^^^^^' 
half the distance; 46027 has vanished altogether. In the 
case of lithium this extinction can be carried further in 
the flame reaction with an ordinary Bunsen burner in which 
the red line 6705*2 is alone seen.^ 

Sodic Chloride, Na CI. — The D line ; ^l^ ^ >• is by very sod^ 

far the longest line in the sodium spectrum ; it is, in fact, 
the longest metallic line I have observed. After D come 

6iCA'2 f *" ^^^ ^^^ ^"^ c68i-A [^" ^^^ yellow, the latter 

pair having a slight advantage over the former. ctc2-c i 

come next, and the shortest is 4982 5, really a double line, 
but so nebulous and ill-defined that Thalen has repre- 
sented it as single. In the chloride we find D 5 00^ I- all 
across the spectrum, and all the others have vanished but 
a trace of S^^r^ j 

Magnesic Chloride, MgClg. — Magnesium has three Magmsic 
lines {b) surpassing all the others in length ; their wave- '^^^^'*^^' 
lengths are 51830, 5172*0, and 5166*7; these lines alone 
arc constant in the chloride; 44810, the winged line, 
sometimes flashes in. 

Zincic Chloride, Zn CI2. — Zinc has three long lines in Zinac 
the blue, 4809*7, 4721*4, 4679*5; these only are visible in '■'*^'^'^*^- 
the spectrum of the chloride. One line, 6362*5, in the 
extreme orange, is of the same length apparently as the 
shortest of the three blue lines, but is not visible, possibly 
on account of its faintness. 

Strontic Chloride, Sr Ch, — Strontium has one extremely strontu 
long line, 4607*5, and this with two in the indigo, 4226*3 '^^^*^' 

* It is necessary in dealing with Li CI and Na CI to have the poles 
rather far apart (8 to 10 millims.), as, on account of the easy volutiiity 
of these chlorides, if the poles are close all the lines appear stretching 
across the spectrum. 



LABORATORY WORK, 



547 



A cursory examination of the spectra of some amalgams 
of tin and magnesium has shown that this is the case. 

For instance, it is possible to begin with an alloy which 
shall only give us the longest line or lines in the spectrum 
of the smallest constituent, and by increasing the quantity 
of this constituent the other lines can be introduced in the 
order of their length. This reaction is so delicate that I 
learnt from it, a thing I had not before observed, that the 
least refrangible line of b, the triple line of magnesium, is 
really a little longer than its more refrangible companion ; 
for the spectrum of magnesium was reduced to this one 
line in an alloy in which special precautions had been taken 
to introduce the minimum of magnesium. 

It follows from this statement that not only is the spec- 
trum analysis almost infinitely more delicate than it has 
hitherto been supposed to be in the case of the elements 
in which the difference between the longest and shortest 
lines is least,^ but that in time it may become quantitative ; 
for if the admixture of certain other bodies extinguishes 
the .shorter lines of metallic spectra, it would seem that a 
series of carefully executed maps of the spectra of alloys, 
the proportions of the constituents of which are known, 
will place in our hands the means of determining (roughly 
it is true), by mere inspection, the quantity of the sought 
metal present in an alloy, the composition of which gad 
that metal is unknown. At the same time it is clear that 
further progress must be made before such a method can 
be practically employed in the arts. 

Although the working hypothesis which has suggested 
the various lines of research which have been followed 
is, I think, sufficiently clear, I refrain from dwelling upon 
it until other researches now in progress enable me more 
fully to judge of its value, and to state at greater length 
the various conclusions which may be drawn from it. 



FOURTH 
PAPKR. 

/iffcct of 
alloyiug. 



Prohtih/r 
quantita- 
tive appli- 
cation. 



> The great lengths of the lines of sodium, lithium, &c., at once 
account for the delicacy of their spectrum reactions. 

N X 2 




t. as is well kncFvn. all 
fd an donat 



ftcAos ii dctaa. it «iU be wen 
t tf KflLttuff and AngstrDm. 
K^^ick OHT present knoir- 
fce ante- Jhtm i h ae, «s detcr- 
i* •<"■*» «hsCTvatioo— that is, 
> (>'' tW nirxxis solar 




LABORATORY WORK, 549 

be the case with calcium, magnesium, and sodium. The fourth 
number of the bright lines in the spectrum of each of these '*'^''^'^' 
metals is indeed small ; but those lines, as well as the dark 
ones in the solar spectrum with which they coincide, are so 
uncommonly distinct that the coincidence can be observed 
with very great accuracy. 

" In addition to this, the circumstance that these lines Kinhhojf's 
occur in groups renders the observation of the coincidence ^^""^' ^' 
of these spectra more exact than is the case with those 
composed of single lines. The lines produced by chromium 
also form a very characteristic group, which likewise coin- 
cides with a remarkable group of Fraunhofer lines ; hence 
I believe that I am justified in affirming the presence of 
chromium in the solar atmosphere. It appeared of great 
interest to determine whether the solar atmosphere contains 
nickel and cobalt, elements which invariably accompany 
iron in meteoric masses. The spectra of these metals, like 
that of iron, are distinguished by the large number of their 
lines ; but the lines of nickel, and still more those of cobalt, 
are much less bright than the iron lines, and I was there- 
fore unable to observe their position with the same accuracy 
ivith which I determined the position of the iron lines. 
All the brighter lines of nickel appear to coincide with dark 
solar lines ; the same was observed with respect to some 
of the cobalt lines,^ btit luasnot seen to be the case with other 
equally bright lines of this metal. From my observations 
I consider that I am entitled to conclude that nickel is 
visible in the solar atmosphere ; I do not, however, yet 
express an opinion as to the presence of cobalt. Barium, 
copper, and zinc appear to be present in the solar atmo- 
sphere, but only in small quantities ; the brightest of the 
lines of these metals correspond to distinct lines in the 
solar spectrum, but the weaker lines are not noticeable. 
The remaining metals which I have examined — viz. gold, 
silver, mercury, aluminium, cadmium, tin, lead, antimony, 
arsenic, strontium, and lithium — are, according to my obser- 

* I'hc italicj* arc mine. 



SOLAR PHVSfCS. 



vations, not visible in the solar atmosphere. Through the 
kindness oT M. Graiideau, of Paris, I obtained several 
pieces of fused silicium ; I was thus enabled, by using 
tlicni as ek'ctrodcs, to examine the spectrum of this 
clement. The lines in the silicium spectrum are, howev-cr, 
witli the exception of two broad green bands at iSioand 
1S30, so deficient in luminosity that I was unable to deter- 
mine their position with sufficient accuracy to reproduce 
llif^Di in my drawing. The two bright green bands do not 
correspond to dark bands in the solar spectrum, so that as 
r.ir ,is I have been able to determine, silicium is not visible 
ill the solar atmosphere," 

It will be seen from the foregoing that Kirchhoff dealt 
in.iinly with the brightest lines, although the test failed 
him in tlie case of cobalt, for a reason I shall show 
further on. Hence, as a result of Kirchhoft's work, uc 
li.ivc in the solar atmosphere : — 

/■/.'^■»/. Doubtful. Ahi,Hi. 

Sodium, Cobalt. Gold. 




LABORATORY WORK, 



551 



Hydrogen. 

Manganese. 

Titanium. 



Sodium. Chromium. 

Iron. Nickel. 

Calcium. Cobalt. 
Magnesium. 

Thus rejecting zinc and barium from KirchhofTs list of 
accepted elements, adding cobalt from the doubtful list, 
and hydrogen and manganese from Angstrom's, and tita- 
nium from his own obser\'ations. 

The table of coincidences referred to, and Angstrom s 
remarks thereon, explain the cause of this. KirchhofTs 
evidence for zinc had depended upon the coincidence of 
two lines only, and these were doubtless thought insufficient, 
as in the cases of the metals retained in the list the 
number of the coincidences was much greater ; viz. : — 



FOURTH 
PAPER. 



Alefals re- 
Jain fd by 
Anj^strom, 



Sodium . 
Iron . . 
Calcium . 
Cobalt 
Manganese 



. 9 (all) 
450 

. 75 
. 19 

. 57 



Magnesium . 
Chromium . 
Nickel . . 
Hydrogen . 
Titanium. . 



18 

33 

, 4 Tall) 

118 



Barium » . . . 1 1 (of 26) Zinc . . . . 2 ? (of 2^) 
Aluminium . . 2 ? (of 14) 

From Angstrom's remarks, which I proceed to give, it is 
evident that he was not quite satisfied with the brilliancy 
test relied on by Kirchhoff, and that his doubts concerning 
zinc arose from this cause. 

" L'aluminium possede certainemcnt des raies brillantes Ansstrciux 
en plusieurs endroits du spectre, mais les raies situees entre oiuhTbrii- 
les deux H sont les seuls qui semblent coYncider avec les UaHcytat. 
lignes Fraunhoferienncs. Pour expliquer ce phinomenc 
singulier, il faut dire que les raies violettes se presentent 
comme les plus fortes dans le spectre dc ce metal. Dc 
mdme que les raies jauncs du sodium, ces deux raies 
d aluminium ont fait voir quelquefois le phenomcne d*ab- 

* I include this " below the line," though I cannot but think that its 
omisbion bv Thalcn was accidental. 



552 



SOLAR PHYSICS. 



FOURTH 
PAPER. 



The lines 

in the 

solar spec- 

irum are 

the longest. 



sorption consistant en ce qu'une raie noire se presente dans 
le milieu de chacune d*elles, ce qui prouve la forte intensite 
des dites raies. En observant les rayons extra-violettes dc 
ce m^tal, on ddcidera si les deux raies mentionn^es cidcs- 
sus coincident ou non avec des raies Fraunhof6riennes ; car 
si ma supposition est vraie, les raies extra-violettes doivent 
cotncider aussi avec les lignes obscures du spectre solairc. 
" A deux raies du zinc que j*ai indiqu^es sur mes planches 
comme coTncidant avec des raies Fraunhof^riennes il en faut 
ajouter unetroisi^me,situ6ea48o9*^; mais, k regard des deux 
raies, tr^s-larges et tr^s-fortes, d*une apparence n^bulcuse; 
il n*y a pas de correspondance visible ; ainsi, la presence 
du zinc dans le soleil me semble tr^s-douteuse. Je dirai 
cependant qu'il existe trois raies de magnesium, du mcme 
aspect n^buleuse, que ne possedent pas non plus dc 
correspondance avec les raies de Fraunhofer, quoique la 
pr&ence de ce corps dans le soleil ne permettre pas Ic 
moindre doute."^ 

In the accompanying maps the lines of certain metallic 
vapours reversed in the solar spectrum are given under the 
spectrum mapped by the new method. // will be seen tkit 
invariably t/te reversed lines are simply those tvhick an 
longest in the spectrum. 

It is not necessary on the present occasion to dwell upM 
the great importance of this determination, both in con- 
nection with the fact just stated and the other facts touch- 
ing the lines which remain longest in chemical combina- 
tions *^ and mechanical mixtures. It supplies us at once 
with the true test to apply to the reversal of solar lines and 
a guide of the highest value in spectrum observations of 
the chromosphere and photosphere. It is one, doubtless^ 



4 



^ It will be seen from my maps that this statement is not accons. 
Thaldn's later work left only one line doubtful. 

*-' A. Mitscherlich has noted the disappearance of certain lines * 
consequence of the presence of several substances in the same to*, 
but he only applies this to the sun by supposing the substances to bt 
combined, and so not to give a spectrum. — Attn, de Chim, €tdtri^ 
3 s<5r. vol. Ixix. p. 176. 



LABORATORY IVDKX, 



L*i 



which will shortly enable ii5 to ceterziine th- z-rss-erire 
new materials in the solar at2no?7her*. 2^f :t is st^sn at 
once that to the last published tabic cf >:l£r e'.^ziert? — 
that of Thalen — must be added, zisc. alj~r;un. zjsd 
possibly strontium^ as a result of the appliztt:::: of the 
new test 

In the case of the chromos:>hcre, the c-bser-.-aticris c f the 
order of lengths of the bright lines is invested TR-ith a zi^rx 
importance, as also the obsen-ation of line? -ahii ^re not 
reversed in the ordinar>- so!ar spectrin. As 2:2 instajiice 
of this, I may state, that the fad that the re-r*r»ers£l inti) 
brightness in the chromosphere of the lint i^fxK is not 
due to iron \'apour, is settled by the other firt. -ah-.i this 
new method has enabled me to deten::::-* that the coin- 
cident line in the iron spectrum is one of the shortest lines 
in the whole spectrum. 

In the case of the photosphere, not only xn^y we hope 
to account for such cyclical changes st^ I have long had 
reason to suspect, and have referred to in prior con:n3'jnica- 
tions to the Society; but it is esseniial that ipot-^pectra 
shall be photographed with spedai reference to the con- 
sideration that in such spectra the new lines may now be 
found, in all probabilit\% to be those which are on'.y slightly 
shorter than those ordinarily re\'er5ed. This revrarch I am 
making arrangements to cany- on. 

It will be observed that in the niar^s the elements arc 
arranged in the order of their atori-ic wc:;^hts. This wa^ 
done before all the comparisons were made, because, as 
I have before announced to the Royal Society in the case 
of several of the elements, the length of the lines in the 
spectra of the vap>ours observed in the chromosj^hcre arc 
also most frequently arranged in this order, as predicted liy 
Mr. Stoney.- The comparison rendered possibUj by the 

" I3arium also, if a lapsus calami has not hicen made. 

* This arrangement has since been broken up {*tT ihe ^ oiivenirnrc 
of the engraver. Some of the speara havin;( Uith sun and rhloridt 
lines had to be displaced by others without tlicsc in order to ;;rt the 
^holc of the map5» on to the three Mates 



f::"ith 



2*74 K w 








1 toKie eiemeiiis wiiii lugnet aiotiiit; wcij 

i />,/ir.^., lines reversed is less. But the maps at 

i h^T^'ani °"^^ ^^^ higher layers of the chrome 

Im^i-iir^M constant action goes on, are passed, ati 

"'' 'J^™" to be a guide ; and we are therefore d 

siderations, which promise to lai^ely ina 

. 1 of the kind of action at work in the sol 

the cyclical variation of that action. 

The maps which accompany this cc 
been made by my assistant, Mr. R. 
have only been revised by myself. I ai 






this opportunity of testifying to the z 
has displayed in a research necessarily 
its character, and requiring great patien 

[It has been found impossible to give 
and tables which appear with this con 
Phi/. Trans. The accompanying Plate, 
some of the maps arc reduced, will s 
results of this research.] A 



LABORATORY WORK, 555 



Fifth Paper. 

Researches in Spectrum Analysis in connection with the 
Spectrum of the Sun, No. II. — By J. NORMAN LOCKYER, 
F.R.S. Received March 14, 1873.^ 

[Abstract.-] 

The observations in this paper are a continuation of fifth 
those referred to in the previous communication bearing _**^'*_^^ 
the same title. They deal (i) with the spectrum of 
chemical compounds, and (2) with the spectra of mecha- 
nical mixtures. 

I. Chemical Compounds. 

Several series of salts were observed ; these series may 
be divided into two : — ist, those in which the atomic 
weights varied in each series ; 2nd, those in which the 
associated elements varied in each series. The following 
salts were mapped : — 

PbFj, PbClg, PbBrg, Pblg; SrFg, SrCU, SrBr2,SrIo; Salts whost 
BaF^, Bad.,, BaBr^.Bal^; MgF,, MgCl^, Mg Br^, '^^'^^ 
Mg Ij ; Na F, Na CI, Na Br, Na I. obsen^^, 

' The conditions of the experiments are described ; the 
same aluminium cups, described in the first paper, were 
used, and the poles were arranged in such a manner that 
they could at will be surrounded with any gas or vapour. 
Hydrogen was used in some of these experiments ; it was 
purified in the usual manner by drying and freeing from 
traces of sulphuretted hydrogen ; it was then passed over 
clean cut pieces of sodium, and admitted to the poles. An 
induction-spark from five one-pint Grove cells was used, the 
circuit being luithout the Leydcn jar, 

* Proc. R. S, vol. xxi. p. 285, No. 144. 

' This paper will appear />/ eAiejiso in the forthcoming volume of 
the Philosophical Transactions. 



SOLAR Phvsics. 

The lead compounds behaved (in air) as follows; — 

The fluoride gave the eleven longest lines of the mctil, 
but four were verj' faiot. 

The chloride gave nine lines ; one of these is very 
short. 

The bromide gave sbc lines, but one is a mere dot oo 
tlie pole. 

The iodide gave four lines distinctly, and two as<lots, 
one of which is scarcely visible. 

It is pointed out that the decrease in length and number 
of lines follows the increase in the atomic weight of the 
. non-metallic element, the lines dying out ia the order of 
their length. 

Barium was next experimented on, the !>ame series of 
salts being used. A marked departure from the resulu 
obtained in the case of the lead compounds was obscr>'c<l| 
especially in the case of the fluoride, its spectrum bcii^ 
much the simplest ; in fact, it consisted of only four Una. 
Strontium behaved like barium, and so did niagnesiutD 
.. fluiiridr. This anomalous behaviour was found to be most 




LABORATORY IVORK. $57 



strontium line appearing in conjunction with an oxide fifth 
spectrum. The strontic fluoride, however, refused to give ^^^^^' 
any spectrum whatever. These results are compared with 
those obtained with the weak spark, and it is shown that 
the difference is one of degree ; e, g, baric bromide gives 
25 lines in the spark ; these are the longest lines. In the 
Same it gives but one line ; but this is the longest of all 
the barium lines, and indeed very far exceeds all the others 
in length. When the flame-spectra are compared with 
those produced by the low tension spark, the spectra of 
the metals in the combination are in the former case 
invariably more simple than in the latter, so that only the 
very longest line or lines are left. 

Some experiments made by Mr. R. J. Friswell to deter- spectra of 
mine the cause of the similarity of the spectra of the ^^inau^ 
irarious salts of the same metal observed in air are then jfiames. 
I^ven, the conclusion being that the spectrum observed 
is really that of the oxide. 

Kirchhoff and Bunsen's, Mitscherlich's, and Clifton and Remits ob- 
Roscoe's prior conclusions on the points investigated are ^'"/Aer^ 
stated at length ; and it is shown that the observations observers, 
recorded, taken in conjunction with the determination of 
the long and short lines of metallic vapours, are in favour 
of the views advanced by Mitscherlich, Clifton and Roscoe. 
For while the spectra of the iodides, bromides, &c., of any 
element in air are the same as stated by Kirchhoff and 
Bunsen, the fact that this is not the spectrum of the metal is 
established by the other fact, that only the very longest lines 
of the metal are present^ increased dissociation bringing in 
the other metallic lines in order of their length. 

The spectra have been mapped with the salts in hydrogen : spectra in 
here the spectra are different, as stated by Mitscherlich ; ^Z^'^f 
and the metallic lines are represented according to the hydrogen. 
volatility of the compound, only the very longest lines being 
visible in the case of the least-volatile one. 

The following are the conclusions arrived at : — 

I. A compound body has as definite a spectrum as a 



SOLAR PHYSICS. 



^^B 


N 


— „ 


■ 


^^H 


_ 




■ 





LABORATORY IVORk', 



559 



simple one ; but while the spectrum of the latter consists 
of lines, the number and thickness of some of which 
increase with molecular approach, the spectrum of a com- 
pcmiid consists in the main of channelled spaces and bands 
whidi increase in like manner. In short, the molecules of 
a ample body and of a compound one are afTected in 
the same manner by their approach or recess, in so far as 
their spectra are concerned ; /;/ ot/ier words, both spectra 
tmve tikeir long and s/iort lines or bands. In each case 
the greatest simplicity of the spectrum depends upon 
the greatest separation of molecules, and the greatest 
oooBplexity (a continuous spectrum) upon their nearest 
appfoach. 

.3. The heat required to act upon a compound, so as 
to render its spectrum visible, dissociates the compound 
aoponiing to its volatility ; the number of true metallic lines 
ihlch thus appear is a measure of the dissociation, and 
doubtless as the metal lines increase in number the com- 
pcmnd bands thin out. 

Mitscherlich's observations, that the metalloids show the 
seme structural spectra as the compound bodies, is then 
referred to, and the question is asked whether the mole- 
cules of a metalloid do not in structure lie between 
tiioee of [metallic] elements on the one hand and com- 
pounds on the other. 

These considerations are applied to solar and stellar 
spectra; the general afppearance of the solar spectrum 
shows tliat in all probability there are no compounds in 
the sun. 

Secchi's maps of a large number of stellar spectra are 
referred to as now indicating beyond all doubt the existence 
of compound vapours in the atmospheres of some stars ; 
and it is suggested that the phenomena of variable stars 
may be due to a delicate state of equilibrium in the 
temperature of a star which now produces the great ab- 
sorption of the compound and now that of the elemental 
molecules. 



FIFTH 
PAPER. 



Dissocia- 
tion of 
compouuii 
molecuhs. 



:lax yarsics. 




deals with mechanioi 
of alloys of the follawing 

of Cd icro, 5-0, i-ovoij. 
Za lot^ 5x1. lAO!. 
:^ i^s^^L^ « Mg iox>. i-o, o-i, CTOi. 

It 5 jvigg-- :vz Aafe Ae liaes disappear from the 
^;Tr.-r-,3i is r:ii ^i^nataee becocDes less, the shoftest 
,(te& :s>icx:ex-.n;L^ ssst ; aad titot. althoogh we have hoc 
nil ur:=*aaL:i:w-j2c ot a^M^ftitM Spectrum anal>-sLs. the 
r.-u^ a* to be napplkahle. It is then 
~ur -^r^-x RSBWcfekoa a neOiod which promises 

?ar:r:; .t" aea c na d fcr <m ov laiowle«%e of the 
'.-c-^ ^ -^ :x 1SK smi% ifaa b ei e is then discussed, 



Bi^^IVV -S ; 




III.— NOTES. 

NOTE A} 
Spot Phenomena and Theories. 

The following pages contain a rdsiutU of the observations notk a. 
and theories of various investigators of the spectrum of 
solar spots. My own observations and theories it will be 
unnecessary to refer to again, as they have already been 
given. 

My first observation was made in 1866 (see p. 435). 
In 1867, Mr. Huggins stated to the Royal Astronomical 
Society that in the spots no certain modification of the 
solar spectrum had been detected.^ In 1868, however,"* 
in a paper read before the Royal Society on May 14th, 
after stating that he had examined the whole spectrum 
from A to G and found no lines in the spots which were 
not present in the ordinary spectrum of the sun, he 
observed that the absorption was both continuous and 
selective, as I had already done, the lines C and F being 
but slightly intensified, while the group of chromium lines 
a little less refrangible than b was especially marked by 
increased strength. The same was the case with D, of 
which Mr. Huggins remarked : " These lines appeared 
slightly broader, as if by the addition of a faint and 
narrow nebulosity at both sides." 

* Sec p. 438. 

' Monthly Notices^ R.A S., vol. xxvii. p. 131. 

3 Phil. TiiVis., 1868, p. 553 ct scq, 

O 



562 SOLAR PHYSICS. 



NOTE A. He further mentioned the lines B, d, E, many of the 
iron lines in Kirchhoff*s map, and probably 20662 and 
2067* I (Kirchhoff), as participating in the increased strengtt 
Father Secchi, whose first idea seems to have been that 
the spots contained aqueous vapour — about which more 
presently — communicated, in 1869, several papers to the 
Paris Academy of Sciences, in which he makes the fol- 
lowing remarks on the phenomena observed : — 

" Lorsqu'on place une tache dans le champ du spectroscope. 1< fc- 
sceau des raies correspondantes se pr^sente k peu pr^s comme sui : 
1° les raies noires qui sent tr^s-fines et tr^s-nettes dans notre instnxmot 
sur le reste du Soleil, paraissent se gonfler et s'^largir k ira%-€rs h 
tache ; leur bords ne sont plus tranches netiement, comme dans le 
teste du spectre ; 2° un grand nombre des raies tr^s-fines ei \ peac 
visible ailleurs deviennent tr6s-larges et ndbulpuses, comme jc Tai dea 
indiqu^ dans une autre communication ; 3° toute Hiarmonie de ImtA- 
sit^ relative des lignes brillantes se trouve profond^ment altinee, 
et, pendant que quelques-unes diminuent ^normemcnt dintensiti, 
d'autres traversent toute la tache et m^me le noyau, sans s'affaibtr; 
4*^ dans celles que paraissent s'affaiblir, cet * effet est dCi plutitt \ oo 
empi^tement des lignes noires ^largies qu'k une diminution rcclk(if 
lumi^re. Ainsi dans la demi6re tache, T^largissement des ric 
D' et D" ^tait si g^nd, que Tinterval lumineux disparaissait prcscae 
compl^tement, pendant que, avec notre fort instrument, clics bxx£, 
tr^s-s^par^es et tr^s-nettes en dehors de la tache.'* * 

" Mais une classe de ph^nom^nes encore plus int^ressantes a inc< 
mon attention : la zone obscure qui se d^veloppe par absorptioo dui 
les noyaux entre les raies D et C m'a fait chercher s'il n'y en avait pa* 
d'autrcs. Effectivement, j'ai constat^ que, dans quatre regions (fc 
spectre, cette absorption devenait plus sensible que dans Ic reste . j' 
Tune de ces regions se trouve dans Ic rouge, pr^ de C du cot^<JeB; 
2" une autre prds de la raie D ; 3° un espace assez vastc dans Ic \tr. 
et ce qui est plus remarquable, j*ai obser\'d que sur le fond dc «cs 
n^bulosit^ sombre brillaient des raies lumineuses, s^parecs dm a 




illigen 

tres-modifi^es ; mais ce ne sont pas les sculcs : il y en a uo gi*' 
nombre d'autres qui le sont dc la m^me mani^re. Ces denx ni0 
appartiennent au calcium. Des ph^nom^nes pareils se d^tloffclt 
dans le groupe voisin du fer, et surtout dans le ^oupe compris ciV 
les raies 1207 et 1241 de Kirchhoff et dans celui dont le milin c*^ 
respond k la raie 142 1 de Kirchhoff. Ces raies deviennent plus iv« 
et restent bien tranch^es. Or beaucoup de ces raies appartksatf 
au fer, et j'en ai identifi^ un grand nombre. Au contraire ks n* 

* Comptes Rendus^ voL Ixviii. p. 764, 20th March, 1869. 

* Op. cit., vol. cit, p. 961, 26th April. 



SPOT PHENOMENA AND THEORIES, 563 



du magn^ium ne sont que tr^s-faiblement influencdes ; les raies du note a. 

sodium qui s*^argissent, mais deviennent ndbuleuses aux bords et en 

quelques autres points, sont peu influenc^es. De Ik, on pourrait conclure 

aue CCS vapeurs sont, k des hauteurs diffi^rentes, en proportion tr^s- 
ivcrses." 1 

He also states that the aspect of the spectrum of spots, 

especially on the penumbras, is like that of certain stars, 

and then that tlie spectrum of a spot resembles that of the 

limb : — 

** Ayant examind comparativement le spectre du noyau des taches 
ct celui du bord du disque, du c6td int^rieur, je suis arriv^ k la con- 
clusion que ces deux spectres se ressemblent consid^rablement. L'^lar- 
gissement des raies constat^ dans les noyaux se reproduit pr^s du 
iKHd, de sorte que, dans cette region, il dgale sou vent celui qu'on voit 
dans les taches les plus Idg^res et les plus supcrficielles." ^ 

" Ainsi se trouve confirmee indirectement cette assertion, que Tab- 
sorption qu'on remarqu^ dans les noyaux des taches n'est pas due h 
des masses ^trang^res qui flotteraicnt au-dessus dc la photosphere, 
mais seulement k une plus grandc profondeur de Tatmosphdre tra- 
verse, car le meme effet se produit pr6s du bord par la simple inter- 
vention d'une plus grande ^paisseur de cette atmosphere meme."^ 

This however he has now (1873) abandoned. 
The next statement is that there are no new lines in the 
spots: — 

** Nous avons vu que, dans ce spectre, il n^ a pas production de 
caies fondamentales nouvelles, mais seulement un renforcement con- 
siderable des 'raies solaires connues ddjk existantes." ^ 

In his description of the spectrum of a solar spot, April 
9th, 1870, Professor Young states : — ^ 

. ..." Many of the dark lines were widened and deepened in this 
nucleus spectrum in the manner which the description and figures of 
Mr. Lockyer have made familiar. Many also were unaffected. Among 
these were notably <7, B, E, 1474, the four lines of ^, 1691 and G. 

•* The two sodium lines D^ and Dj, and 850 (Ft) were distmctly, but 
not greatly, widened. 

** The effect was most marked upon the following : — 864 (Ca). 877 
CFe ?), 885 (Ca), 895 (Ca and Li), 1580 (Ti), 1599 (Ti), 1627 (Ca),'and 
1639 (Ti). I have marked 877 doubtful, because there lies very near 
H a line whose origin is unknown, and I am not sure to which of the 
two the thickening was due. The Titanium lines are identified as 



r 



* Comptes RenduSy vol. Ixviii. p. 962, 26th April, 1869. 
J Op. cit, vol. Ixix. p. 40, 5th July. 

4 Op. cit., vol. cit., p. 41, ?th July. 
Op. cit, vol. cit., p. 166, 19th July. 

• Journal of Franklin Institute^ No. 533, p. 64, July 1870. 

002 



SOLAR PHYSICS. 



■ ihe^ 



n Angstrom's Alias. I was greatly sarprised at Uw 
iissume in the spoi-s-pectrum, as ihey aie uKoa- 
nml spectrum ; and a siinilar remark applies to tin 



icuous in Ihe 

'■ 1 do not inttnd to cfinvey tire idea that the lines meniioned "Mt 
e Linly ones thjt vtrc much deepeoed : there were many oihcn, 
■' ' " " ' to nearly the same degree, but I had not time W 



Ji.-nlil\ tln:m.' 

liut it is 

Uuunt Slier 

lie states :- 



to th. 



briUiant results of his expedition to 
that I am most anxious to call attention. 



It spectra of 5e\ era! different spots were carefully studied, ind a 
^tii' was drawn up of 155 lines which are more or less affeacd, 
y by being grt-.itly widened, but in some cases by a wcakeniE^ 
.rial, bevtr.d bright lines were also found in the spot-spcctnim, 




SPOT PHENOMENA AND THEORIES. 565 



With regard to the reversal of the Calcium lines H^ and note a. 
H*, he adds : — 

'' It was found that these two lines (not the hydrogen lines, as has 
been erroneously reported) are also usually, and I am pretty confident 
always, reversed in the spectrum of sun-spots, not so clearly, more- 
over, in the nucleus as in the penumbra, and over a somewhat extensive 
region surrounding it. This reversal of the H lines does not involve 
at all the disappearance of the dark shade, but a bright streak rather 
than a line makes its appearance in the centre of the shade, which 
itself is, if anything, a little intensified." 

Prof. H. C. Vogel ^ gives the following list of lines seen 
in the spots : — 



660-4 


Fc 


599-0 


6531 




5948 Fe 


65 15 




very much thickened. 589*5 ) Na 


6476 




588-9 ] (D) 


646-2 


Ca 


587-4 


6454 




5856 Ca 


6449 


Ca 


5789 Fe 


6430 


Fe 


579-0 Fe 


6411 


Fe 


5785 


6407 


Fe 


5762 Fe 


6346 




575-9 


641-4 


Fe 


575-4 Fe 


629*4 




573-0 Fe 


6293 




572-6 


629* I 




571-7 


629*0 




5708 ) 


6282 




5705 \ Fe 


6280 




570-I ) 


627-6 




5681 / ^^ 


6240 




6239 




5664 


6237 




566-1 Fe 


6236 




5658 Fe 


6I9I 




518-3) 


617-2 


Fe 


517-2 ^Mg 


6 1 6-8 


Ca 


5167) 


616-2 
616-1 


Ca) 
Ca( 


ver>' much thickened. ^X-L Fe 


614-4 




491-8 Fe 


613-6 


Fel 




612-1 


Ca! 




6io'i 


CaJ. 


much thickened. 


606-4 


Fe 




600-7 


FeJ 





* H. C. Vogel, " Beobachtungen angestcUt auf der Stemwarte des 
Kammerherrn von Biilow zu Bothkamp." Heft i. 1872. 



oLAM pj/rs/cs. 



s had coarinccd himftelf that theie j 
c betwcea the ordinar>- solar and s 
Ates:— 

oaaUa some of the co«ulitiona of the solir 
■noneKa cteened may have been produced 
indvODOn by wUch the lines of absoiptitf 
■iSHk aw Qdiatkm from the gas itself, and n 
cHiiMfAji bJiiid the absarpiion by the gai «f 
Thia caitif would produce increased btadc- 
** " ml for more than a slight appk* 

T breadth of the lines seems a 
s in which their power of absoip- 
led rai^c of uate-lcngth. TmI: 
a rf fMCS varies in thb nrspect is shown Ij-^ 
wfacb aamK of (be brishi liiKs of some guca 
oodilMtts of tefUtOD uid temperature, ll «B, 
be gqw iwio a of the lines of hydrogen as lte| 
JdKT ioCRMe in the nngc of its po»e( iC 
mC dUmgh ft slmutd take place under simibr 



«cf density in the vapaon 
Soch a stale of things would ncccss*nl» 
t Bcaxer the sun's ceruie ; but we do not 



SPOT PHENOMENA AND THEORIES, 567 



" Enfin je crois m^me avoir vu dcs traces de vapeur d*eau dans le note a. 

floleil, et surtout dans le voisinage des tachcs. On voit Ik les mSmes 

sMes de raies n^buleuses que lorsque les cirrus vont traverser le 
champ de la lunette. Mais cela demanderait k ^tre appuy^ par des 
nouvelles observations." ^ 

^ Dans la demi^re communication . . . . je vous annoncais C|ue je 
croyais avoir constat^ la presence de la vapeur d'eau dans le voisinage 
des taches solaires, en ajoutant toute fois que cela demanderait des 
recherches nouvelles. Ayaiit eu derni^rement de belles journ^es, j'en 
ai profit^ pour analyser le phdnom^ne, et je vais Texposer en detail 
vu sa grand importance." 

«««««« 

** IVapr^s ces r^sultats, il parait clair que la vapeur d^eau existe dans 
Tatmosph^re solaire, au voisinage des grandes taches : il rcste seule- 
ment k verifier la Constance de ces phdnom^nes et si elle se vdrifie 
pourtous, car j'ai rencontrd des taches tr^s-petites et tr^s-noires qui ne 
la pr^entaient pas." * 

Alluding to certain nebulous lines in KirchhofFs map, 
he says : — 

** On ne connait pas les substances que les produisent, mais quel- 
Ques-uns d'entre eux sont dus sans doute k la vapeur d'eau, surtout 
dans Torang^ et la jaune."< 

At the same time that he was promulgating these views 
as to the presence of water in spots, he was defending the 
selective absorptive nature of the phenomena from the 
attacks of Professor Respighi, who considered it might be 
accounted for instrumentally : — 

** Le ph^nom^ne que nous vcnons de signaler n*est done pas I'cffet 
d'une simple diminution de lumi6re dans le fond, mais il est bien dO 
^ une faculty absorbante dective plus intense, qui reside k I'intifrieur 
des taches. En effet, nous savons que les taches sont des cavit^s 
dans le photosphere : dans leur int<5rieur, la couche absorbante doit 
^re plus ^paisse, et alt^rer beaucoup de rayons que ne sont pas 
absorb^s par latmosph^re extdrieure. Les lignes brillantcs qui tra- 
vcrsent souvent les noyaux pourraient bien etre les lignes directes de 
CCS gaz que j'ai signal^s comme constituant la masse gazeuse in- 
t^rieure du soleil d^s le mois de Janvier 1864 (voir * Bull. Mdtdor. 
de rObservatoire/ 31 janv. 1864, vol. iii. p. 4). 

" Cet effet serait alors compl6tement semblable 2i celui qu'on observe 
dans Tatmosph^rc terrcstre, ou une couche plus dpaisse pr^s de Thorizon 
produit r<5largissement de lignes fines, et donne k un grand nombre 
d'autres lignes, h, peine visible quand le soleil est asscz haut, un 



' Comptes Refidus, vol. Ixviii. p. 238, ist Feb., 1869. 
' Op. cit., vol. cit., pp. 358—360, 15th Feb. 
• Op. cit., vol. cit., p. 1084, loth May. 



p JMK s^ Ih^ Sk 



sag widi M. Fajrc I 
actiom as t^ prime [■ 



followii^ 



of front 

" lo in qoesi' open [hb 
CTVoiMii nd cxntro dcUe mu 
ha bbogno, di prore pin pa 
la naccfaia i famiata princ 
prowemeoti dalla eraziaiu, 
poco a poco rcgolariziand 
unto che in progrcssQ di I 
tnostra rosservaiionc."' 



The 



fsMm^im 



SPOT PHENOMENA AND THEORIES. 569 



" M. Faye continue : * Les aches ne sont plus que le receptacle dcs note a. 

I>roduits trop lourds pour ^tre entrain^s par I'hydrogfene au-dessus de 
a chromosphere.' 

"Ce n*est pas Ik Texpression exacte de ma pensde : les taches sont 
bien, selon moi, dues k des amas de vapeurs dmiscs par Eruption et 
refroidies ; mais je n'ai pas dit que Thydrog^ne ne pouvait les 
entrainer, ni qu'elles ne pouvaient d^passer la chromosphere. Com- 
ment aurais-je pu le penser ayant vu le magnesium et le sodium 
^ev^s jusqu'k une et quelquefois deux minutes. Habituellement 
m^me, dans les masses vives, on les voit ddpasser considdrablement 
la chromosphere. Je n'ai pas davantage considdr^ ITiydrog^ne comme 
^tant le vdhicule des autres vapeurs : j'ai dit seulement que les vapeurs 
m^talliques sortent m^Mes k rhydrog^ne, mais je crois qu'elles pour- 
raient ^alement sortir par leur propre force, et sans Taide de ce gaz." 1 

M. Rayet considers that though the spots may owe their 
formation to an ascending current, according to M. Faye's 
first theory, yet that a descending current is the more 
probable. That if the hot atmosphere surrounding the cloudy 
photosphere rushes down upon it, the clouds will dissolve, 
and the brighter portions will then appear as a facula ; and 
then if the hot current penetrates still deeper, the photo- 
spheric rain will cease, and be replaced with a bed of clouds, 
thus causing a penumbra, the centre of which being still 
hotter, stopping all condensation, will produce a spot 
nucleus. He then adds : — 

"Cette demiere hypoth^se en faisant depend re la formation des 
taches de Taction d'une couche ext<5rieure k la photosphere, montre 
parfaitemcnt que les facules doivcnt prdcdder les taches et se trouvcr 
accumul^s autour d'elles : ce dont I'hypothese de la cause interne, ne 
rend que difficilement compte." 

He adds that we have direct proof of these descending 
currents, and refers to Secchi's observation of the promi- 
nences inclining towards a spot, and to my own of the rush 
of the willow leaves into a nucleus. He then adds : — 

" Si en effet le noyau dcs taches (5tait formd d'un nuage de particules 
solides froides, ce noyau sera it absolument obscur ; or, Tanalyse 
spectrale de cette rdgion m'a montrd que le noyaCi poss<5dait une 
lumiere propre tres-sensible et que, s'il <5tait moins brillant que la 
masse de la photosphere, cela provenait d'un accroissement dans 
Vobscuritd des lignes noires du spectre solaire ordinaire, accroissement 
identique k celui qui r^sulterait d'une augmentation dans la densitd des 

1 Comptes Rendus^ vol. Ixxvi. p. 912, April 14th, 1873. 



SOLAJt PHYSICS. 



lapcurs mdtalliqvies qui reitfennc I'almosphire solaire ; or, c«i prf- 
cisi^mcnt un accroisscmcnt de cet utdrc que doit praduire la disNluUtii 
des niiages photo sph cliques par un courajil chaud."' 



Mk. Hlggins and the New Method. 

Soon after I began to observe the sun telescopically, is 
iSOi, with my 6| object glass {a cluf-iftcuvre made by 
Messrs. Cooke, of York) which I received in that year, 
I hat! the pleasure of making the acquaintance of Mr. 
(now Sir William) Grove, who informed me that for many 
years hi; had been attempting to observe the solar pro- 
minences by means of red glass in ordinary sunlight,, 
stopping out the sun's image in the telescope by a suitable 
diaphragm. When, therefore, I published my paper in 
iS66, in which w.is contained the idea of rendering them 
visible by means of a spectroscope, I looked upon the red- 



MR. MUGGINS AND THE NEW METHOD. $7^ 

In February, 1868 — nearly a year and a half after my note b. 
paper — Mr. Huggins presented an account of the work 
done in his observatory to the Anniversary Meeting of the 
Royal Astronomical Society. After stating that he 
had succeeded in constructing a new spectroscope with 
dispersion equal to about seven prisms of dense flint- 
glass of 60**, which, be it remarked, is more powerful than 
the spectroscope I have usually employed in my researches 
up to the present time (1873), ^he report proceeds : — 

** Mr. Huggins intends to make use of this instrument in the continua- 
tion of his observations on the spectra of different parts of the sun's 
surface, and of the solar spots. He has already insulated the spectrum 
of the umbra of a spot. During the last two years numerous obser- 
vations have been made for the purpose of obtaining views of the red 
prominences seen during a solar eclipse. The invisibility of these 
objects at ordinary times is supposed to arise from the illumination of 
our atmosphere. If these bodies are gaseous, their spectra would 
consist of bright lines. With a powerful spectroscope, the light 
reflected from the sun's edge would be greatly reduced in intensity by 
the dispersion of the prisms, while the bright lines of the prominences, 
if such be present, would remain but little diminished in brilliancy. 
This principle has been carried out by various forms of prismatic 
apparatus, and also by other contrivances, but hitherto without 
success." 

In October 1868, 1 was fortunate enough, as described in 
p. 446, to obtain a .spectroscope which, although of less 
dispersive power than the one then in use by Mr. Huggins, 
enabled me, on the first day of its use, to observe the lines 
of a solar prominence. 

escape all other methods of observation at other times ? And if so, may 
we not learn something from this of the recent outburst of the star in 
Corona.^' 

" I gave this advice to my friend, Mr. Lockyer, because I thought that, 
9S it might be some time before he obtained the new instrument, it 
might be well that he should publish what I conceive would enable 
him to claim for himself the knowledge of this principle. And I think 
that anyone well acquainted with spectra, on reading the question put, 
could not fail to see what was meant ; and, if he were previously ignorant 
of the principle, he could not fail to perceive it. I therefore feel rather 
astonished that anyone should claim the statement made by Mr. 
Huggins two years afterwards as being the commencement of a new 

?rinciple." — iMter of Dr. Balfour Stewart, in Nature , vol. vii. p. 301, 
ebruary 20th, 1873. 

* Monthly Notices, R.A.S.^ vol. xxviii. p. 88, Feburary 14th, 1868. 



\L' HicTJis - '^' 




t; -,•: f i3 ii t:;:' 






fa fl'wBirfBJBpaattowhidt Huffc 

: .im f p agk «< this ■ Sotc* 
tj Oftc 3B3y al cfciA (ran M-Janssa 
i>3^id Id 9CwI a vcplT. » the ioanta* 

1 ifd as k vas BBwoctlrjr- Here it is 






ofdv 

• Tbe vr?~ix.-. n>=^- - ^ af &e nagnb s Att «y wtA was tnicif 
s «is am IB^ tie oha ma boaa Bade dunof 

i jaiw M A«« — i. Thu mr «wt 



:>rt - !-■ 



MR. MUGGINS AND THE NEW METHOD. 



in to clamp on another part and work back, and then to begin 

"II. We now come to the second point. In what I have hitherto 

■ted I have shown thai the coincidence in lime between my own 

iults and the receipt of the information from India ■was due lo the 

te OH which the new iHstrumettt wai received. I am convinced that 

the new spectroscope been received a year ago, the discovery 

Id have been made just as easily ; and Father Secchi's remark on 

ease of the observation, even with an instrument of moderate dis- 

^ power, strengthened this view. I have also slated my method 

of work. 

let us see how the Indian observations were the cause of my 
kuccess, or if indeed they could have caused the success of anyone 
who depended upon them. 




" On the Joih October, I knew of three sets of observations : 

ayet's, Herschel's, and Tcnnant's. 

"Rayeigave B D E * F 

id four more undetermined lines. 

■* Herschel gave j jj''^^''^, [ D \'^T \ 

blaring against F, and hesitating very much lo assign even an 

}proxiiii3te place to the line in the red. 

Teonant gave CD * i F 

Sieved he also saw a line ne^ir G, 

" The above table will, I ihink, show how utterly useless the Indian 
iHCrvations were for the purpose Mr. Huggins has assigned to them. 
1 bci, every lettered line, except A and H, is named. Had 1 indeed 



fl 



574 



SOLAR PHYSICS. 



NOTE B. worked with them at all, I should naturally have looked for the line D, 

as that is the only line in which all the observations correspond. 1 

distinctly referred to hydrogen (= C and F) in my paper of 1866. bnt 
in 1868 only one observer is sure of C, and only one obser>-cr is 
sure of F ; and now it is stated that my success is due to the Indiaa 
observations. 

** I have sent this paper to the Royal Astronomical Society, not because 
any credit due to M. Janssen and myself is lessened by Mr. Hugging 
paper, but because I know from experience that its Monthly Sotica 
are one of the most important sources from which the history of 
astronomy is written. I naturally, therefore, prefer that a true accooBt 
of the recent discovery should appear in the Afonthly Noitus, iostod 
of a statement incorrect in fact, which, were it left unnoticcxi, wouid 
mislead those who come after us." * 

Up to the present time — I state it with regret— Mr. 

Huggins has neither substantiated his injurious statement 

nor withdrawn it 



NOTE c. 



e 



NOTE C.2 
Absorption at the Limb. 

At p. 205 will be found a quotation from Angstrom 
referring to Forbes' observation on this subject In my 
paper (p. 478), although I did not state the position ia 
the spectrum of the selective absorption which I held to 
be possible, I was of opinion that a selective absorptioc 
in the case of the violet rays at least, was almost ceruit 
from the fact that actinically the difference between liib 
and centre is apparently greater than it is visually. Tbi 
idea I mentioned to Mr. Rutherford some time ago. but be 
properly reminded me of the very great influence pbotK 
graphic processes might have in producing the effect I 
had noticed. 

It is nevertheless, I think, true; and Herr Vogd te 
recently made some detailed observations of great intetst 
bearing out this view.' 

In addition to this absorption in the violet, Mr. Hasti^^ 
of the Sheffield Scientific School (needless to add U3.KI* 
recently succeeded in obtaining evidences of the incftisfll 

J Monthly Notices y R,A,S,, vol. xxix. p. 4, 1869. 

See p. 478. 
» See Naturforschery Jahrgan^V,, p. 321, Oct. 1872. 



ABSORPTION A T THE LIMB. 575 

selective absorption in the visible part of the spectrum, note c. 
Upon comparing the spectra of the centre and the limb, 
he remarks : — ^ 

" Certain differences are recognized besides that of intensity, by far 
the most marked of which are exhibited by the lines b^ and b^ which 
become sharper and less hazy near the limb. The line b^ possesses 
the same characteristic, but to a less degree ; C and F also become 
sharper in the same region. Excepting these and the D lines, it re- 
quires very close examination to detect any variation. There is, how- 
ever, a line in the red at 768* i of KirchhofT's scale which is strongly 
marked near the centre of the sun's disc, but disappears entirely, to 
my power at least, within 16" to 20" from the limb. Two other lines 
below F, at 1828*6 and 1830*9 of the same scale, exhibit nearly com- 
plementary phenomena, />., they are strongly marked near the edge, 
but much fainter at the centre. These latter lines also become greatly 
strengthened over the penumbrae of spots. The line 768*1 is not thus 
atfected. These are all the differences which I have invariably seen 
repeated in examinions since February 17. 

" Others have, however, been suspected. Certain lines, which arc 
strengthened in a region of spots like those above mentioned, appear 
to be strengthened also near the edge, but do not undergo so marked 
a change. It is obvious that the differences should be most pro- 
nounced in the clearest sky, and such is the case. The closest ex- 
amination has extended only from B to a short distance above F, as 
the plate glass of which the small prism is made has a decided yellow 
tint, and absorbs the blue rays strongly. 

" Since the light from the border of the sun undergoes a general 
absorption, which reduces its intensity to much less than one-fourth 
that at the centre, according to Secchi's measurements, and yet the 
spectroscopic character is changed so slightly, it is impossible for 
me to escape the conviction that the seat of the selective absorp- 
tion, which produces the Fraunhofer lines, is below the envelope which 
exerts the general absorption. But the phenomena of the facula: 
prove not only that this envelope rests upon the photosphere, but 
also that it is very thin. The origin of the Fraunhofer lines, then, 
must be in the photosphere itself, which is in accordance with Lockyer's 
views. 

"Any effects which the chromosphere might produce we would 
anticipate finding most evident in the lines of those gases which are 
readily detected there. A reference to the observations shows at once 
a compliance with this anticipation in the lines of hydrogen, magnesium, 
and sodium. The line 768*1 is not less strikingly in concordance, if 
it be regarded as 768*?* (the ? indicates doubt as to the tenths of the 
scale, and * absence of a corresponding black line) of Young^s Cata- 
logue of Chromosphere Lines. The lines 18286 and 1830*9, with 
others of the same class, probably have their origin in the medium 
which exerts the general absorption, and thus are allied to our telluric 
lines. It also seems probable that the chromosphere is too transparent 

* Nature^ vol. viii., p. 'jj, May 22nd, 1873. 




construction of instruments w'as conc< 
ever, postponed this branch of the rcse 
Mr. Hastings has taken it up. 




of sollt image Hiploycd 



' ddUiHK o( a iront ^ 



In addition to the instrument figure 
was also early suggested by ZolJner, I 



^^^= 



ABSORPTION A T THE UMB, S77 



Mr. Hastings' method of obtaining the same result is note c. 
thus stated by himself : — ~ 

** I have constructed a small prism with four polished sides, its bases 
being parallelograms. This is so placed that one face rests upon the * 

slit-plate of the telespectroscope, and has its acute edge perpendicular 
to the slit at its middle point. The instrument may then be directed 
so that the image of the sun falls with its centre on the uncovered 
portion of the slit, while the light which forms the edge of the sun, 
nlling perpendicularly upon the first surface of the prism, suffers two 
interior total reflections and a displacement depending upon the form 
of the prism. A glance at the figfure, in which s s' is the slit, L V the 
diameter of the sun's image, and P the prism, shows that no light 
firom the covered part of the slit will reach the collimating lens except 
that which has been reflected from the two sides of the prism. The 
relation of the acute angle {v) and the distance between the reflecting 
sides (/) to the focal length of the great telescope {F) and the width of 
the spectrum {a) is given by the formula, 

2/ sin t/ = F tan 16' - a. 

The sides of the prism not fixed by the equation admit of considerable 
latitude, but should be made to approach the lower limit in order that 
the planes of the direct and transmitted images may be as little sepa- 
rated as possible. Of course / and 7/ should be so proportioned that 
-^he reflections may be total. 

** When the instrument is properly directed and in adjustment, we see 
very narrow black line dividing tne spectrum longitudinally into two 
s of widely different intensity ; the fainter, belonging to the limb 
the sun, is marked on its edge by the bright chi*omosphere lines. 



H- 



Fig. 160 —Mr. Hastingn' arrangement for comparison of spectra. 

In the apparatus described, two similar prisms were also placed 
z-w the slit in a synmietrical position. The spectra of two opposite 
f «rs of the sun were thus brought together, and the change in refran- 
' " ity due to the sun's rotation was very clearly shown." * 



* Nature, May 22, 1873. 



m 


■ 


^r maktng obsei^ 
H ances. 1 
B In my first j 
procured by (^ 
prominence, a 
varying length ■ 
gave an idea ot\ 
tremely narrow, J 
obvious that in d| 
could not be se« 




^^^^H 


1 


■ 


■ 


rapidity to allowd 
set Janssen and 
Janssen by gjvin 
spectroscope, I bj 
in which I was £ 
panded it. , 

"It occurred to ifl 
ftlorton, that by interi 
which should move wl 
■ng portions of the spe 
Mr. Clark has devise, 
arraogenjent by which 
and diaphragm is cflet 
in tig.6i (p. ,67). 

Hut I find that, alt 
very bright ; yet the w 
oscillation of the eqnl 
details, ™ 


r 




1 



METHODS OF VIEWING THE PROMINENCES. 579 

we wish to observe, than to move the solar image by the tangent screws note d. 
of the equatorial/* » - 

To ZoUner belongs the credit of having first indicated 
the open slit method now generally adopted. 

This physicist, in a paper communicated to the Royal 
Saxon Academy of Sciences on February 6th, 1869,' makes 
the following remarks on the method of seeing the promi- 
nences without an eclipse.* 

" With a moveable slit, the brightness would be 

diminished in proportion to the distance travelled over by the slit. 
Especially in the rotating spectroscope the brightness of the protuber- 
ance itself would decrease from the centre of rotation to the edge, and 
thus prevent the observation of the natural brightness of its parts. 

** For this reason I intend to employ another ver>' simple arrange- 
ment. 1 have convinced myself of its practicability by the experi- 
ments on terrestrial sources of light described below."* 

'* The principles upon which it is based are as follows : — 

" I. The apparent brightness (lustre, claritas 7'isa) of a protuberance 
is independent of the opening of the slit, provided that it retains an 
appreciable breadth on the retina. 

"2. The brightness of the superposed spectrum increases in propor- 
tion to the width of the slit. 

" 3. In oscillating or rotating slits the brightness of the superposed 
spectrum remains unchanged, while that of the image of the protu- 
berance, produced by the permanence of the light, decreases according 
to another law, depending on the number and duration of the impres- 
sions on the retina, and on the refrangibility of the observed band of 
the protuberance. 

" If, for simplicity, ve suppose that the whole surface over which the 
slit travels in its rotation or oscillation is filled with the protuberance, 
and that the intensity of the produced secondary- image is in inverse 
ratio to that surface (corresponding to a uniform diffusion of the light 
over that surface through a stationary slit), then the relation of the in- 
tensity of the background to that of the protuberance would remain the 
same whether we first decrease the brightness of the protuberance by 
means of oscillation of the slit, and thereby leave the brightness of 
the superposed spectrum or background (according to 2) unchanged, 



1 Nature^ Decemoer 8th, 1870. Extracted from the Journal of the 
I^ranklin Institute. 

• Published in No. 1772 of the Astronomische Nachrichten, Sept 
15th, i860. 

• The translation quoted is by Professor Mayer, and appeared in the 
Journal of the Franklin Institute^ vol. Ixxxviii. p. 410 et seq. 

• These consisted in making the images of a petroleum lamp flame 
and that of a spirit lamp, the latter coloured with a salt, coincide on a 
reflector, and then observing and separating the combined images by 
means of the arrangement proposed. 

P P 2 



58o SOLAR PHYSICS. 



NOTE D. or secondly y open the stationary slit so widely that the opening extends 

- '- over die space travelled over by the oscillation of the slit in the first 

case. By this means (according to i ) the apparent brightness of the 
protuberance remains unchanged, while that of the background is 
mcreased in the same ratio in which it was diminished before vith a 
constant background. 

'Mf these suppositions are correct, the end in view can be much more 
easily obtained in the second way, if we take care not to admit the 
intense light of the body of the sun into the slit. 

*' It is only necessary to open the slit so far that the protuberance, or 
a portion of it, appears in the opening. By polarizing or absorbing 
media, placed before the eye-piece, the light in the whole field of tiev 
can be so diminished that the proper relation of intensity between the 
protuberance and the superposed spectrum may be obtained.** 

On the i6th of the same month in which Zollner com- 
municated his ideas to the Saxon Academy Mr. Muggins 
sent a paper to the Royal Society of London, in which be 
announces a method of observing the forms by means of 
a combination of an open slit and of ruby glass.^ 

" A spectroscope was used ; a narrow slit was inserted after the 
train of prisms before the object-glass of the little telescope. This 
slit limited the light entering the telescope to that of the refrac- 
gibilily of the part of the spectrum immediately about the bright br.e 
coincident with C. 

" The slit of the spectroscope was then widened sufficiently to adnu 
the form of the prominence being seen. The spectrum then becaoe 
so impure that the prominence could not be distmguished. 

'* A great part of the light of the refrangibilities removed iu froo 
that of C was then absorbed by a piece of deep ruby glass : the 

prominence was then distincdy perceived A more detailed 

account is not now given, as I think I shall be able to modify the 
method so as to make the outline of these objects more easily visible. 

On the 29th I heard of this paper of Mr. Huggins\ and 
it at once struck me that the absorptive media employed 
by him were useless, which is still my opinion, aod I 
communicated this to the Royal Society on the 4th of 
March (see p. 482), proposing to use merely an opeo slit 

On the publication of Mr. Huggins' paper it was sea 
that his proposed arrangement set all optical prindpies at 
defiance, but I did not point this out until the publicatiofl 
by him, as notes to a translation of Dr. Schellen's work 
on Spectrum Analysis, first, a statement that the d^ 
scription given in the Proceedings of the Royal Sociity »^^ 

Pcoc, R,S,y vol. xvii. p. 302. 



METHODS OF VIEWING THE PROMINESCES. 581 

erroneous [why had it not been corrected ?], and secondly, kote d. 
that the method employed by him was identical with the 
one employed by Zollner and myself!^ 

Since this time Mr. Huggins has corrected his paper,' 
and I print the correction in a foot-note' that its value may 
be estimated, and that he may have the benefit of it so far 
as this work is concerned.^ 

In May 1869 Captain Herschel suggested the use of a 
prism of red glass ^ without a slit, and used the open slit 
method with success. 

Whether the narrow or wide slit be used, it is impor- 
tant that the image should be enlai^ed by the spectro- 
scope lenses and not by the object-glass of the telescope 
employed to throw the image on to the slit This is 
accomplished by having the observing telescope longer 
than the collimator (see Fig. 85, and Zollner, translated 

* "Schellen," pp. 423 and 425. 
« Proc, RJ^,^ voL xxi p. 127. 

* ** When editing the English translation of Schellen's * Spectrum 
Analysis^' I discovered that the short account of the method of view- 
ing the forms of the solar prominences by means of a wide slit, which 
I had the honour of presenting to the Royal Society on February 16, 
1869,* does not agree exactly in one respect with the account of the 
observations of February 13 as it was entered at the time in my Obser- 
vatory-book. The short note was written at the suggestion of a friend 
during a Conmiittee held in the Royal Society's apartments, and, as 
the concluding words show, was intended to be followed by a more 
detailed account of the method of observation. The point in question 
relates to the position of a second slit which was used to screen the 
eye from every part of the spectrum except that under observation. 
The words in mv book written at the time are ' narrow slit found to 
be best at focus of little telescope with positive eyepiece.' In the note 
the second slit was stated to have been placed before the object-glass 
of the little telescope. Such an arrangement was tried in connection 
with some other experiments in progress at the time. The plan of 
limiting the field of view to the part of the spectrum corresponding to 
the refrangibility of the light of the prominence, as well as the employ- 
ment of a ruby glass, is of value when the air is not favourable, or 
when a spectroscope of small dispersive power is used.*' 

^ The remarks of mine which called forth the correction will be 
found in The Academy ^ vol. iii. p. 290. 
» Proc, R,S., vol. xvii. p. 508. 

♦ Proc, R,S. vol. xi'ii. p. 302. 





Fio. iSi.—Ctll wilh priioi filed. P ui the pKui, lu Parii uMhn 
nng; lu refractun angre is ii'; a and a, nailni - ' " 
niBuw of the Ktcwi * and i i c ii a holEto lacililal 




METHODS OF VIEWING THE PROMINENCES. 

by Meyer, Journal Franklin Institute, vol. ZZ, p. 415, 
and Young, fournal of the Franklin Institute, No. 550, 
p. 348. Nov. 1 87 1). 

In addition to the methods above described, which all 
depend upon the use of the spectroscope in its ordinary 
form, others have been proposed. One method consists 
in placing a prism of small angle before the object-glass 
of the telescope, another in cutting the curves of an object- 
glass on the faces of a prism, another in placing a direct 
vision prism in front of the slit. 

By means of these arrangements the slit of the spectro- 
scope, situated in its ordinary position, could be immersed 
in light of any required refrangibility, the dispersion being 
increased to a lai^e extent. In the case of eclipses it is 
of course possible to entirely do away with the slit, as 
the dark moon covers the sun's disc, while the corona is 
thus practically its own slit. 

During the present year (1873), Mr. Seabroke and 
myself have communicated a method to the Royal Society 
which enables us to observe the whole of the chromosphere 
at once by means of a ring-slit : — ' 

" The observations made by slitless spectroscopes during ihc eclipse 
of Dec. u, 1871, led one of us early this year to ihe conclusion 
that the most convenient and labour-saving contrivance for the daily 
observation of the chromosphere would be 10 photograph daily the 
image of a ring-slit, which should be coincident wilh an im.igc of [ho 
chromosphere itself. 



® 



Fic. i6}.— Diapkngm showing annulus. Ihe bieadih of whtch m 

"TTiesamc idea has since occurred to the oihep 
"We therefore beg leave to send in a joint 
Koyal Society on the subject, showing the mar 



' Proc. R.S., vol. xxi. p. 105. Read Nov. 6, 1872. 



1 



STRt/CTUftE, ETC., OF THE CHROMOSPHERE. 



, The accompanying solar profili 

[ the dates stated, by nitans of the 

by the authors at the meeting : — 




) 



ur knowledge that 
IS, but had rejected 
Professor Winlock, in America, has Iried a similar arrangetnent, 
'but without success.— J. N. L.. G. M. S., January 17, 1873.] 



Structl'kk. Height, and Composition ( 

ClIROMOSI'lIEKE.' 



I add a note, ; 
sphere, entered 
1 809 r — 



of the edge of the chronio- 

■vatury-book on March 13, 




"Yesterday 1 watched the chromosphere by placing the slit at a 
. tangent, and to-day got rather a different result. Yesterday I got a 



SOLAR PHYSICS. 



E E. billowy look. To-day, however, it was exactly like the edge ot ven 
ragged clouds." 




The following are Professor Respighi's observations on 
this point, which entirely endorse my description at the 
page quoted : — 

"In my Note III. at page it), I said : 'The red stratum, ot itir.> 
sphere of incandescent hydrogen, which envelopes the solar bodi. 
while appearing at its base to be regularly terminated in a dciu!;^ 
arch, at the summit presents an irregular aspect, that is. at tucei 
terminating in points, or filaments either vertical or inclined, at t:mci 
rounded like cloudy mounds, and finally undulating like mounuis 
ranges. But very seldom (and ihis in the locality of the spoti :'. 
presents the appearance of long, regular strokes, even to the summ.!. 
resembling a bright circular arch. 

" ' The height of this stratum varies, but generally it doii nut ei 
cccd 12'. Its brilliancy also is variable in different parts of iht si L: 
outline, and at different times, and is generally greatest in lh<; luc^:- 
of the spots." 

"' The height of this stratum is variable in the socn. 

parts of the solar circumference, and generally seems higher in ih: 
vicinity of the poles than at the equator, and on rare occasions 1: .> 
found to be very low near the large groups of protuberances."' 

On this point Secchi's statements are as follows:— 

" Elle se prijsente sous quaire aspects bien tranchi's. 
" {lij Le premier aspect est celui dune couche nettcmcni lennii-.T 
comme serail la surface librc d'un liquide. Son dclat tranche parfi.ii- 
mcnt ^ I'ext^rieur avec I'cspace sombre cnvironnant ; on lenungic 
sculement une faible diminution d'intensitt* pr^ du bord ext^rieor. 

"(b) Ordinairement, la chromosphere est gamie de petits fiUmffl:^ 
somblables ^ des polls brillants, dirig^ dans un mcme sens, plus ot 
moins inclines. Cette structure s'observe suriout cntre les biftinb^ 
moycnnes et les poles. L'entrainement dea filets n'est par toujoan 
dirig»5 dans le sens des courants sup^eurs, qui transportent ks pn- 
tubt'ranets, mais cela arrive tr^s-souvent. 

" ('■) Quclquefois, surtout dans les rdgions des facules, la suffice >v 

diffuse {sfiimafa), dc mani^re <iu'il est difficile de dire oii elle s'airtw. 

"(</) Enlin, Ic plus ordinairement, la chromosphere est iefBi:>- 

irri5guli£remcnt et garnic dc pctiis appendices conitjucs im^w^ 



PROMINENCES ON THE SUN 



ou de petites flammes dirig^s en tous sens. Ce sont des protuberances 
rudimentaii^s, plus fr^uentes dans les points du p<5rim^tre solaire ou 
se presdntent les granulations ou marbnires de la surface ; de sorte 
quil parait exister une d^pendance entre cet <5tat de la chromosphere 
ct les granulations. 

^On pourrait distingiier ces quatre aspects de la chromosphere par 
les noms de piaie^ veltu^ diffitse (sfumata) et flamboyante.^ > 



587 



NOTE E. 



NOTE F. 
Prominences on the Sun.^ 

I was at first under the impression that it was not 
probable that prominences would be seen on the sun itself 
(p. 479), and so was Father Secchi {Comptes Rendus, vol. 
Ixviii. p. 237). We were, however, both mistaken. 

My first observation of these was made on the nth of 
April, 1869. Secchi's letter to the Paris Academy announc- 
ing the same appearance is dated the 13th, so that the 
observations must have been nearly simultaneous. 

Father Secchi's first observations are thus recorded : — 

" Mais ce qui faisait un dtrange contraste avec cet aspect gdndral de 
toutes les raies, c'^tait la raie C de Thydrog^ne, qui s'cffa^ait com- 
pl^tement presque partout, et surtout dans les p^nombres. 

"J'ai mis la fente de Tinstrument perpendiculaire au pont qui 
traversait la tache, et alors j'ai vu trois csp6ces de spectres bien 
tranchdes dans le m^me champ de vision : 1° le spectre solaire 
ordinaire, 2^ celui des taches avec les lignes noires et les bandcs 
rcnfonc^es ; 3° les raies de Thydrog^ne, disparucs presque partout 
sur la p^nombre, mais devenues brillantes sur le pont ct sur la partic 
des noyaux la plus voisine de lui. Je ne saurais expliqucr cet^cnsemblc 
de phdnom^nes par de simples changcments d'intensitds : il y a 
^videmment ici une absorption tr6s-puissante, et un renversement de 
raies, qui s'effectue manifestement aux points corrcspondants aux voiles 
rouges existant dans Tintdrieur des noyaux. On a voulu, il est vrai, 
attribuer \ une illusion d'optique ces voiles roses, mais il faudrait ctre 
bien mauvais observateur pour s'y mdprendre : leur aspect contourn^, 
en filets tr^-minces et entrelac^s, sumt pour les distinguer des franges 
dues ct I'aberration chromatique, ct en faire reconnaitre la rdalitd. Ces 
voiles ne sont done autre chose que les protuberances rouges, comme 
je Tavais ddjk dit : aujourd'hui nous avons en outre demonstration 
directe du renversement du spectre. " ' 

* Comptes Rendus, vol. Ixxiii. p. 827, October, 1871. 

« See p. 488. 

' C R.J vol. Ixviii. p. 960, 26th April, 1869. 



NOTE F. 



SOLAR PHYSICS. 

The reference is to the former part of the same pipa, 
where Secchi describes the appearance of the spot, in whick 
these phenomena were observed, as seen by nteani of a 
polarizing eye-piece. He observes," L'int^rieurdesDoj-vu 
^tait plein de voiles roses contoum^ et encbev^tre dt 
toutes manieres " (p 960). 

Again : — 

"A 6 heures la raie C est tris-forte partout snr le diique mUit, 
niais elle disparatt complement daiu le vouinagc du nojrau, wi b 
pdnombres,"' 

Captain Herschel observed the bright lines of hydrogni 
on the sun's disc as early as June loth, 1869.* 

In a communication to the Franklin Institute, cbtcd 
October 3, 1 870. Professor Young states ■ that in a group d 
spots then visible, the C and F lines were frcqucnLr 
reversed. His account runs as follows : — 

" At 40J P.M. the brilliance of the F line increased so grtuh lbs 
it occurred to me to widen the slit, and to my great delight I siff aya 
the disc of the sun itself a brilliant cloud, in all ils stiuclurr W 
detail identical with the protuberances around the limb. InitK. 
there were two of them, and there was no difficulty in tracing out i:^ 




delineating iheir fonn. Fig. 168 reprtsenls them as they w«nfo= 
4.05 to 4.10; Fig. i69gives theform at 4.15-ia They »-efe tber. ..t 
siderably fainter than at firsL During the intervening ten minirt* . 
examined the other lines of the spectrum, and found that the t~" 
could be distinctly made out in all the hydrogen lines, even in ^ ^ ^ 
that the reversal of the other lines, including D„ was confiMd t«^ 
region immediately over the spot-nucleus, where the smaller but bci^ 

' Comptes Rendiis, vol. Ixviii. p. 1089, loth Mav, 1869. 
' Pioi. R.S., vol. xviii. p. 64. 
' XntHtf, vol. iii. p. 113. 



PROMINENCES ON THE SUN. 



cloud tcnninated abruptly \ or, I might better say, originated. The note f 

larger one faded out at both ends, When the clockwork of the equa- 

lorial was stopped, the luminous cloud took 167 seconds of time to 
traverse the slit ivhich was placed parallel to the hour-circle. This 
indicates a length of at least 130,000 miles, without allowing anything 
for ihc foreshortening resulting from the nearness of the sun's limb. 




*' I may add that in the telescope this group of spots, from 

their first appearance, exhibited a strong yellowish tinge, which ap- 
peared to over-lie all the central portion of the cluster. So conspicuous 
was it, that several persons, unaccustomed 10 astronomical observation, 
noticed it <it once before 1 called their attention to it. The penumbra 
of rhe group was unusually faint." 

M. Rayet makes the following statement wilh regard to 
this subject: — 

" Mardi dernier, iz Avril, 1870, en examinant au spectroscope la 
lumiire d'une immense tache, centre d'un groupe irts-^endu, situ^ 
dans la region sud-ouestdudisquesolaire,j'aivu la hgne C se tenverser 
et devenir lumineuse dans la portion rifpondant au noyau. 

" Je ne connais qu'une seule observation analogue : celle faite par Ic 
R. P. Secchi, le 11 Avril, 1869, ct meniionn^c en detail dans les 
CompUs Rendus du 6 Sepiembre, i86g. 

" Les circonslances que rendent possible le ' ' 

dans une lache sont done peu fr^uentcs." ' 



NOTE G. 

Changes of Wave-i.ength. 

In no part of recent solar research (except in that re- 
lating to spots) has such disagreement occurred, both as to 
facts and the interpretation of them, a.s in relation to the 



1 



' Compies Hindus, vol. Ixx. p. 846, 1870. 



4 



590 SOLAR PHYSICS. 

NOTE G. changes in position of the lines described by me immc- 
diately after the discovery of the new method, and ascribed, 
on tfte groufid of the selective nature of t/u phenomati 
to the approach or retreat of the vapours to which th; 
particular lines in question are due. 

Professor Respighi, whom I shall first quote, admittcc 
from the first the reality of the phenomena, but objectcc 
to the explanation of them : — 

"Admitting that the refrangibility of the various luminous r?.;. 
depends upon the length of the wave, as deduced by Fizcau, Bii: 
and others, then if the source moves rapidly towards the obscn^ 
the length of the wave is lessened, and the refrangibility increased 
on the other hand, if the source of light moves rapidly from *i. 
obser\'er, the refrangibility must be diminished, because the w... 
are lengthened. 

" The observations of the protuberances incontcstably show iha: .: 
some jets the hydrogen escapes from the sun with frightful \cloc::>,i 
times not less than 500, 600, and even 700 kilometres a sec:-: 
and I myself have often observed this extraordinary- velocity', esfeciil 
in the jets near to the dark spots. As these jets arc tume<i :r. - 
directions, it may therefore occasionally happen that on the foia: : ' 
cumference also there are similar jets found, either directed !■>*-. 
us, or in an opposite direction. 

" These phenomena, announced by Lockycr, Sccchi, and others. » - 
often observed by me, in every case in the jets near the Iocj!:- 
spots. 

" Portions of the chromosphere near spots arc often obsen-cd n > 
peculiarly bright, and from them intensely bright and more or tv 
violent jets are seen to arise. On placing the slit tangential i. '"^ 
limb, I have often observed at these points ver>' sensible enlar^T.r"' 
of the lines, especially of C, to which I have generally din^t-.-i ~ 
attention. The line C was also occasionally seen to be im--— ' 
being displaced towards the red or the violet, with a more C: . • 
sensible undulation. 

" After the development of the jets, on bringing the slit upon :rr 
in parts (and this near to their base), the brightest streaks irciL:^ 
by the slit, instead of appearing as lines, were presented occa5^.':u- 
in the form of lines broken at angles more or less obtuse. r» 
appearance, however, comes out in a much less marked nur'v 
when the slit is narrowed ; since then, those bright streaks »tTe p^ 
sented each time nearer to a general alignment with C. With tbe > 
widened, it is often found that the line C of the red stratum, or d '-y- 

f)rotuberances, does not coincide exactly with the corresponding bi*t 
ine of the solar spectrum. Such phenomena are also oosenrd i= ^ 
case of Fj and often on a larger scale. In the spectrum of the scm 
parts of the solar disc, especially in the vicinity of the spots, I - ' 
often observed new formations and local enlargements of the dni > 
C, while in other tracts the line itself appeared remarkablv le>*"<"- 



CHANGES OF IVA VE-LENGTH. 591 

refined, or blended, and occasionally totally invisible, or transformed note g. 

into a bright line. In these circumstances, the line C, losing its '- 

ordinary regularity of form, offered the aspect of an irregular line, 
more or less sensibly distorted. Arranging the slit perpendicularly to 
the solar outline in coincidence with the most intense jets, I have often 
observed the bright line C noticeably widened at the base of the jets, 
and not exactly coincident with the corresponding black line of 
the solar spectrum, and the streaks of jets comprised by the slit, 
arranged irregularly in relation to the same hne, or not exactly coincid- 
ing with it ; but even in this case, the irregularities diminish with the 
narrowing of the slit. 

** Seeking the explanation of these phenomena, I have not considered 
it necessary to have recourse to the changes of refrangibility produced 
by the approach or the recession of the masses of hydrogen, in con- 
formity with the principles above alluded to ; but I have found, on the 
other hand, that at least the greater part of them could have been pro- 
duced by other causes — namely, by the strong intensity of the light ot the 
jets; by the influence of the opening of the slit; by the inexact accom- 
modation of the slit plate to the focal distance of the instrument ; from 
the state of agitation of the atmosphere, or from scintillation ; and, 
finally, by the position of the eye in relation to the diaphragm, or to 
the axis of the eye-piece. 

** The points, or bright streaks, which are present on the red stratum 
in proximity to the spots, and by which more or less gigantic jets are 
generally developed, are so brilliant, that they sometimes resemble 
images of the stars, and produce, on the outlines of the slit, bands 
of irradiation and of diffraction, through which the spectral lines 
appear more or less irregularly widened. 

When very fine and bright jets are to be seen, the irradiation and 
the diffraction, apparently lengthening their images, give to them the 
appearance of bright, irregular streaks, more or less inclined to the 
normal direction of the respective spectral lines. These appearances 
become very marked when the eye-piece is not exactly accommodated 
to the distance of distinct vision, and when the opening of the slit is 
not very small. 

"When the slit is not very narrow, the atmospheric spectrum 
becomes diffuse, and on the dark zone corresponding to the black lines 
C and F, a line or black streak is obtained in a more marked way, 
which does not correspond to the central line of the slit ; and therefore 
the bright lines of the chromosphere or of the protuberances not 
coming out symmetrically in relation to that false spectral dark line, 
appear to us more or less sensibly displaced from their normal 
position. 

" If the eye-piece is looked through very obliquely to its axis, the 
very bright lines become easily diffused or doubled, in a way which 
seems as if, besides the normal spectral line, there must be a second 
one of greater or less refrangibility. 

"^\^lenthe atmosphere is much agitated, or when the scintillation is 
rather decided, the stale of aj^itation of the bright lines and their 
irregular variations of intensity contribute to produce in them the 
illusion of greater or less irrej]fularity in relation to the spectral lines. 

" Finally, it is to be remarked that, to produce the above-mentioned 



592 



SOLAR PHYSICS. 



NOTE G. appearances, an actual widening of the spectral lines may greatly 

*- contribute (on account of the very elevated temperature at wlucb we 

find the bases of the very luminous and violent jets). 

'' The irregularities which are presented in the lines C and F, in tbe 
spectrum of some parts of the solar disc, especially in the kxality of 
spots, Le, the enlargement of the lines themselves, their thinning. 
and lastly, their tot^ disappe^nce or transformation into bright 
lines, are certainly due to the influence of jets or protuberances a- 
isting there. 

"In the locality of the spots, the hydrogen proceeding from tbe 
violent and gigantic eruptions remains occasionally suspended (or a 
long time at a ^^reat height, becoming invisible before returning to tk 
sun, and formmg real clouds of dark hydrogen. These clouds of 
hydrogen, acting then purely as a medium of absorption, nuyterr 
well strengthen and widen the dark lines C and F in the sab^aceot 
parts of the photosphere. 

" On the other lumd, if the slit is placed upon a protuberance or a 
very brilliant jet, its brightness may we^en, or destroy, or ncimafize 
the dark line ; so that if the jet is sufficiently bright and the light cf 
the photosphere sufficiently dim, the dark line may become bri^— a 
state of thmgs often observed in regions near the spots and in their 
penumbrae. 

" I do not know whether these explanations may extend to all the ph^ 
nomena observed by Lockyer, but I hold that with all methods they 
ought to be remembered before having recourse to a cause so uncenaio 
at all times as that of the changes of refrangibility or the displacements 
of the spectral lines produced by the movement of the source of lij^ 
" The question of the influence of the movement of the source of li^^ 
on the length of the wave certainly is not so easy to decide as it has 
hitherto been thought ; and therefore it is deserving of deeper study 
and discussion before we can accept as true the consequences abott 
mentioned, and apply them to the explanation of these partiaiUr 
phenomena presented by the protuberances. We must ask for a 
rigorous demonstration of them, or a plausible confirmation, based 
upon direct and conclusive experiments. 

" This question of optics has a certain analogy with that of the 
propagation of light through a refiracting medium in movement : an^ 
while the first concerns the possibility of verifying the approach or 
the recession of the stars, and of the masses of incandescent hydrc- 
gen carried by the solar eruptions, by means of the changes of rtfras- 
gibility, and the corresponding displacements of the spectral lines; 
the second concerns, on the contrairy, the possibility of verifying or 
rendering sensible the movement of the refracting media, tbzvifEh 
which we observe the stars, by means of the differences between the 
aberrations corresponding to the various media traversed by huniaooi 
rays. 

^ In the year 1859, I have shown, by means of suitable cipeR- 
ments, that the aberration is independent of the medium throofk 
which we receive the luminous rays, and that therefore it is impossible 
to explain in this manner the movement of our refracting media, or 
the annual motion of translation of the earth, in which they neces- 
sarily participate. 



CHANGES OF WAVE-LENGTH, 593 



** Those differences of aberration resulted from the hypothesis that note g. 

the rays or the luminous waves were propagated through the medium 

in motion, without suffering any influence, any deviation, or displace- 
ment of this movement. But as it has been demonstrated by experi- 
nientthat these differences of aberration corresponding to the different 
methods do not take place, it must necessarily be concluded that they 
are compensated by a contrary effect produced on the light or 
luminous wave by the conditions of movement with which the 
medium by which they were promoted was invested. 

" The lengthenings and the shortenings of the himinous wave, by 
reason of removal or approach of the source of light, arc deduced 
radically from the tacit supposition — from the hypothesis — namely, that 
the movement of the source of light, or of the centres of vibration, does 
not produce any other alteration in the conditions of the surrounding 
ether ; and therefore the suspicion is not excluded, that in this case, 
also, no conditions of compensation can take place through which the 
luminous wave, in spite of the movement of its source, may arrive at 
the observer of its normal length — />., that which it would have if the 
source were, in relation to the observer, in the condition of rest. 

" This question is of the greatest importance, and as it probably may 
not be definitively determined a priori — that is, by means of the theory, 
I hope it may come to be resolved a posteriori^ by means, that is, of 
experiments more direct and more conclusive than those furnished by 
the solar eruptions." ' 

In his fifth note Professor Respighi seems to imply that 
he .now admits the changes in the position of the lines as 
being independent of conditions. He writes : — 

** Under these masses, and above some jets cmerj^ing from them, 
I have often observed in the lines C, D3, and F those enlargements 
and displacements brought to light principally by Lockyer and by 
Muggins, and which by them is attributed to changes of rcfranj^ibility 
due to the velocity or motion of the luminous gas in relation to the 
observer." ' 

It is therefore impossible to say whether he yet admits 
the explanation which ascribes these phenomena to motion. 

The following are Father Sccchi*s observations on the 
subject. It will be seen that he from the first also admitted 
the facts, but suggested that the cause is the rotation of 
the sun, and not the motion of the gas as an independent 



* Respighi, Note iii. § xii. pp. 31 — 37. The references to Respighi's 
work throughout these notes are to his papers entitled ** Sulle osser- 
vazione spettroscopiche del bordo e delle Proluberanze Solari fatte air 
osscrvatorio dell' Universit2l Romana sul Campedoglio." Extracted 
from the Aiti ddla Rcale Accademia dci Linen. Note v., May 5, 1872. 

• Respighi, Note v. § ix. p. 63. 

n o 



SOLAR PHYSICS. 

body Speaking of a cloud visible above the solar limta j 
on tlic 4tli April. 1870, he writes: — ' 

"En l:i rt^.ird.int avec rimage agrandie, on vil loutc la lif[M 
briUrtnie miii^e st pnijeier, non pas sur Ia raie noire dc ratinosphiit 
CKt^ricnrc ^111 bi>ril sot^iire, inais loute emiire sut \e cb\i lumincui 
plus rclV.iiij^iblL', l.il^s,mt une ligne noire du cbti du rouge exirianf. 
C'l-l.iii coiiiine s\ l.i 1 .110 C eisX cu une r^lrangibiUt^ plus gtandc que U 
ra[c nnire qii'oii m\.in iljiis )e champ ext^cur :iu burd solaire. 

" Nqus ciions di>iii- ^ii face d'uo dc t-ts inysI^ruK changwnents dc 
ri^rangibiliu' quUn .1 i;\pliqu^ pair Ic Unnsport Ct par la force dc 
projcctiiiii ill' I.I iii-iiii re solaire Unc^ avcc une viiewe tftiorme. 

" M.ii- I II 11 il. lii-.int bien celtc fnis, ie con(;usdcs doutessurrate 
cxpliL-iihii.. 1 11 LrrL.r. pour produire le displacement tibsen-*, qui AM 
au nifiin-. /,-, cU II .!;•.(, ince entre D' t-t U* du sodium, ce nuatge auraii 
dii p.ircouiii 111 in in- joo kilometres par Ktoncic : or, conune on nt 
\n\-M\ \\ .^i.i.iri II 1 <|i>i nliiuenlSl cellc mawc. comnirm unaginEr trnt 
tdii- vit, s-( ( i II ijLciion? El, dc plus, comment concilicr ceiw 
hyooilus. ,iiLi. Li position relativemeni ^ b uchc, c|ui ^lait r«$i^l3 
miSnic .111 nil. Ml- iiL-'iidant six heures? Pour adraeitre tout ctla, il 
.' foule de circonstances tr^-improbables, siaon 
L\iye.iil au moins un extreme reserve. 
nm m'ii montrf qu'd nV avail 15 autre chose qu'aiK 
ilri.' de la rotation du Solcil. Ln cffct. cctte ;i»[ie 
.ircourl en un jour un angle dc Sdj minutes, ^tsi- 
r hcure.ou 36 secondcs dnns nnr niinuie dc temps 
o", 6 dans une sccondedetcmjis. Or une seconidi 



impiisiiiblcs, ci 



CHANGES OF WA VE-LENGTH, 595 

de la Terre, mais, dans le cas de la communication, on doit prendre no te g. 

la seconde hdliocentrique, qui est de 3 '^", 4 ; d'ou rdsulte que I'arc 

parcouru en une seconde de temps correspond h, I'^tendue de i "*. 
92, ou presque 2 kilometres. Cette diminution de la grandeur du 
mouvement ne d^truit pas Fexplication du ph(5nom6ne dont il est 

?uestion ; au contraire, elle explique la petitesse du ddplacement, car 
4$tait pour moi une objection que de voir un ddplacement si petit 
dans les raies, tandis que, avec le fort instrument que j'emploie, il 
devait etre beaucoup plus sensible. 

"Jevous prie d'ins^rer le plus tdt possible cette rectification dans 
les Comptes Rendus, m^me si quelque autre avait d^jk relev^ cette 
^uivoque." ^ 

It will be observed that it is now stated that the true 
motion of two kilometres per second accounts for the 
smallness of the displacement observed. In Secchi's first 
communication he had stated, that to produce the observed 
displacement, which was at least ^^oth of the distance from 
D' to D", a motion of 300 kilometres per second was re- 
quired. 

One of his later statements to the French Academy 
on the question is the following : — 

"J' ai vu aussi la dilatation de la raie C en forme de lozange telle 
que la d^crit cette autcur [myself], ct les distortions des lignes D3 ct F, 
que je d^irais vivement verifier." - 

In the following quotation it will be perceived that 
he tacitly admits the justice of my explanation. The 
pamphlet from which it is taken is a reprint of a paper 
read by him before the Pontifical Academy Nuovi Lincei 
on the 7th of May, 1871. The italics are my own : — 

" Vcdendo la vivacita de' gette, restrinse la fessura, e trovai che la 
linea C si mostrava ondulata, ii che indicava una violenta translazione 
iUlla massa. Alia basa de' gette si vedevano molte altre righe, e 
specialmente la rossa a 045 di distanza da C verso B. Nel giallo Dj 
SI aveano dei getti, ma molto piu bassi, e mancava la parte nebulosa. 
La nube era piii viva, e piu aha nel bleu, dove cguagliava almento il 
rosso. La linea viva F rcstava tutta intcra dal lato meno rcfrangibilc 
della nera corrispondcntc." ' 



* Comptes RenduSf vol. Ixx. p. 1062. 
J /ftrV/,vol. Ixxii. p. 306, March, 1871. 
" SuUe Protubcranze solari e le Facole." Nota del P. A, Secchi^ 
1871, pp. 5. 6. 



SOLAH PHySICS. 



Finally, in 
explanation ;- 



1 7 2, he fully accepts my original 



rrari. nv-m i^^istant, en faUant le dessin de la belle Udie 
Ibte siir I:. (Usque, le 13 courant, ^ )oh. 43in. (el dcet. 

■ copic', 5 .iper^ut qu'une langiie tris-tive de feu i-ci^. 
odviiii; .111 milieu &a groupe des qu^tre no)-aus pniidpaa& 
.'iv.-icili.' de sa liiRui;re ^tail telle, q-u'dlc surpassaii, >n' 
s dii double, tOUte le reste du disque dii Sofeil ft da 
cs c-nvironnaalJB. Malheurcusemenl le ciel ^lait saa€' 
u.i:^cs, r(ui enljitJiaient iin travail cl unc efidc contmu* J 

pendant le Wli^ <l"i f«l cmployi! h faire ce desMI^ 
;i-tlrre i>enda|Wi mimues environ.'b Ungue changeade 
1:; CI prit raqJMt'd'iui globule tr^bti Hani, qui paiaissat 
vi5 du Tond te'<lisquc solairt rend;uit cctte transfix* 
>ii, il ]>^init subir un faiblc d<5placanem. ^ 

^Ti^Mii'it que jcfus inform^ du ph^Domine, j'cus recourstf 
ni'ii r,|n-, ti.ledirigcant sur Inplttcc indiqi><!«. jecons "'' 
.■nii>iuiii lu's-vive. LTiydrtJgine pr^seniaii sos raiw rtOh 
.- 1. 1 iviK' C ftajt tr^-bnllUDic, mats ce qull yn depln^ 
npiLbl', test que la parlie la plus vive n'^Liit pns en c<3IK 
t^iTi .ner \:\ rate noire, mats s'tn tfcariait oblitjueni«t , 
111' li.tn^ la figure d-contrv ou la partje ponctoifc ind'HOt 
irLniii reiuersfc Le difplaectnent tftait cans k sens del* 

■ cnii3-.,inle. Ce phi*nomtl«; dijmontiail la projCtliOft 



CHANGES OF WA VE-LENGTH. 597 



double formde de la ligne de Thydrog^ne et d'une ligne un peu plus note g, 

rdfrangible du fer. Au bord du soleil la ligne F de Thydrog^ne se 

renvcrsc et la ligne du fer devient un peu diffuse, en sorte que 
rinter\'alle que, sur la disque solaire, on peut distinguer entre les 
deux lignes, s'efface, et que F parait bordd d'une bande noire du c6td 
du violet De cette apparence pcut, je crois, naitre Tidde d'un 
transport." 

M. Rayet then refers to my announcements of the 
motion of the hydrogen masses in the sun {Proc, Roy. Soc, 
vol. xviii. p. 74), and says — " De parcils observations, lorsgu' 
elles nont pu itre v&ifii^es ni par le R, P. Secchi, ni par 
moiy peuvent-elles cire admiseSy ou biai y a-t-il des viotifs 
de les rejeter comme entachdes d'errcurs f He then pro- 
ceeds to treat the gaseous masses in motion in the sun by 
Bernouilli's formula for the flow of a gas into a vacuum 
and from the calculation obtains the following : — 

" Vitesse d'dcoulemcnt de Thydrog^ne dans le vide h. la surface du* 
soleil. 

Temperature du gaz. Vitesse par scconde. 

o^ 7892 metres. 

1000" 17036 „ 

2000"* 21352 „ 

3000° 27315 „ 



" Les conditions d ccoulement d'un gaz a la surface du soleil sont 
certaincmcnt difft^rentes de cclles que suppose la formule de IJcmouilli. 
Ndanmoins, les nombres prdcddentcs, plutot trop forts que trop faibics, 
car, dans la rcalitd, les gaz no s'dcoulent pas dans le vide, donnent, sui- 
vant une rcmarque de M. Wolf, une idee de la grandeur des vitesses 
avec queJles les gaz peuvent sc mouvoir dans Ic Soleil, et on voit qu'il 
est difficile, impossible m6me, que cette vitesse ddpasse 30 kilometres 
par seconde." 

The extreme rates observed by me on the sun are about 
163 kilometres per second. Young has seen greater velo- 
cities. 

M. Rayet then proceeds to discuss the construction of 
my spectroscope : — 




ou 

rapport ^ „ , 

I'exp^rience journali6re ma prouvd que dans ces conditions de 
dissymdtrie de la vision, le moindre ddfaut dans I'objectif ou dans 
Toculaire se traduisait par un duplication des lignes brillantes ou par 
une distortion de ces lignes. Ces ph^nom^nes singuliers disparaissaient 




■"Cest du restc I'opin 
ticularit& vues par M. 
observation. Voici com 
de l'Obs«r\-atoir« du CoU<! 
ces mouvements et ccs < 
M. Lockycr dti avoir obs 
mem des inouvementB s 
duplication dc la raie, t 
atmosph^ et h la chaleu 
prodoire des d^viaiit 
phdnombies disparaisseni 

The following are ail 
as early as 1869 on U 
the various solar lines j 



CHANGES OF WA VE-LENGTH. 



599 



In a very few moments, a brilliant spot replaced the knobs, not merely note g. 

interrupting and reversing the dark line, but blazing like a star near 

the horizon, only with blue instead of red light ; it remained for 
about two minutes, disappearing, unfortunately, while I was examining 
the sun's image upon the graduated screen at the slit, in order to fix 
its position, which was at — 82^, about 43" from the edge of the limb, 
about 15" inside of the inner edge of the spot-cluster. 1 do not know, 
therefore, whether it disappeared instantaneously or gradually, but 
presume the latter. Fig. 171, ^, attempts to give an idea of the appear- 
ance. When I returned to the eye-piece, I saw what is represented at 
Fig. iJiyC and c\ On the upper (more refrangible) edge of F, there 
seemed to hang a little black mote, making a barb^ whose point 
reached nearly to the faint iron line just above F. As given on 
Angstrom's atlas, the wave-length of F is 486*07, while that of the iron 
line referred to is 485*92 (the units being millionths of a millimetre). 
This shows an absolute change of 0*15 in the wave-length, or a fraction 
of its whole amount, represented by the decimal '00030, and would 
indicate an advancing velocity of about 55*5 miles per second in the 
mass of hydrogen whose absorption produced this barbed displacement. 

"The barb continued visible for about five minutes, gradually resolv- 
ing itself into three small lumps, one on the upper and two on the 
lower line, Fig. 171, </. In about ten minutes more, the F line resumed 
its usual appearance. I did not examine the cline, as I did not wish to 
disturb the adjustments, and risk losing some of the curious changes 
going on under my eye. 

** After the close of this strange phenomenon, I examined, with our 
large telescope of 6-inches aperture, the neighbourhood in which this 
took place, and found a very small spot exceedingly close to, if not 
actually at, the place. This was at 2*45. At 5*30 it had grown 
considerably." 

Not only have we an observation of a change of wave- 
length, but Professor Young s first glimpse of bright lines 
on the sun's disc. 

A year later we have further evidence in this direction 
from the same observer : — 

** Several spots have been carefully examined at different times : most 
of them, in their spectra, gave evidence of unusual disturbances ; 
but by far the most interesting phenomena were exhibited by a large 
group which was first observed near the E. limb on September 19th, 
1870. Changes of wave-length were frequent in its neighbourhood. 

" Figs. 172 and 173 represent the appearances assumed by the F and 
C lines respectively, at the times indicated below each figure, during an 
observation on the afternoon oi September 22nd, 1870. The point 
where these changes of wave-length occurred was at the western edge 
of the penumbra. At other times similar changes were observed, but 
not so great or rapidly varying." * 

' Journal of the Franklin Institute^ and Xature, vol. iii. p. 1 12. 
See also Journal 0/ the Franklin Institute^ No. 535, p. 65, July, 187©. 




" -^lany more or/ 
Aougft none on quid 
On sereral n^«^Z5 



--■""(;.. «oue on quai 
On sererai ocea^SS 
•ne ejecied n>a««^ 
distortion of the hiS 
nearly jjo mjjes Jjfl 
on the surface of SS 
[arge spot." ' ^tI 

The foIJowing^l 
this point : they j 



CHANGES OF WA VE-LENGTH. 6oi 



betnigen 0*23 Milliontcl millimeter Wellenlange, was einer Gesch- note c. 
windigkeit von Ca 20 Meilen in der secunde entspricht.' 



}} 



Dr. Vogel has also seen and described that drift in two 
directions often seen in neighbouring spots : — 

" Am 6 Mai untersuchten wir, Dr. Lohse und ich, spectroskopisch 
einen grossen Sonnenfleck, dessen Kern durch zwei Lichtbriicken 
gespaltcn war. Als der Spalt des Spectroskops auf diese Lichtbriicken 
gestellt wurde, zeigten sich Verschiebungen der Spectrallinien und zwar 
so, dass langs der Kante des grdsseren Flecks die Linien mehr nach 
dem Violctt, an der Kante des kleineren Flecks dagegen mehr nach 
dem Roth gcriickt waren. 

" AufTallig war es, dass sammtliche Linien im Spectrum, so weit es 
untersucht werden konnte,an diescr Vcrschiebung thcilnahmen, deren 
Grosse in der Nahe der F Linie zu 0*05 Milliontcl Millimeter Wellen- 
lange geschatzt wurde. 

" Die Geschwindigkeit des Emporsteigens der gliihenden Gase am 
Rande des grosscrcn Flecks wiirde dcmnach 4 bis 5 Meilen in der 
secunde bctragcn haben." * 

For the mathematical treatment of this subject see, 
in addition to Professor Clark Maxwell's paper referred to 
on page 202, Young in yournal of the Franklin Insti- 
tute, No. 562, p. 349, November, 1872. 

Professor Zollncr has devised a spectroscope, called by 
him a " Reversion Spectroscope," ior the special purpose 
of observing these displaceo^Mj^of the spectral lines, 
and obtaining accurate measof^hiehts of their amounts. 

This is effected as follows : — 

** The line of light produced by a slit or by a cylindrical lens is in 
the focus of a lens, which makes the rays parallel as in other spectro- 
scopes. Then the rays pass through two amicfs systems of prisms, d 
z*ision directe, which I have received from Mr. Merz, Munich. 

** These are so placed side by side that each gives passage to one 
half of the rays coming from the object-glass of the collimator, but so 
that the refracting edges lie on opposite sides. By this means all the 
rays are separated into two spectra of opposite direction. The object- 
glass of the telescope, which unites the rays again to one image, is cut 
perpendicular to the horizontal refracting edges of the prisms, as in 
the heliometer, so that each half can be micrometrically moved, either 
parallel or perpendicular to the line of bisection. By this means we 
are enabled not only to make the lines of both spectra coincide, but 
to place the two spectra side by side instead of superposing them (so 
that one is placed beside the other like a nonius) and also to partially 



* " Beobachtungen angestellt auf der Sternwarte des Kammerherrn 
von Bixlow zu Bothkamp." — Heft. i. 1872, p. 36 and 39. 



SOLAR PHYSICS. 



Uy this coiutruclion the sensitive nature of the 
ot nnlv iiimed to accoant for delermtning any chmgE 

•■IiLi.lr.il lines, bid every juch chanje is aiso lijuilid 
^b 1 .11 IS in oppoMte directioos in the Iwo spectra. 

"1 ilio ycversiou of the speitra which forms the basis 
, .iiiil oliich determines me to give it the name <rf 
'>.'/. , rnay also be applied without the use of the 



List id Chromosphebic Lines. 

My first list of lines^ was, very shortly after it was pub- 
iished, confirmed by Professor Young, who added one 
more to it, 25815 (Kirchhoff ). 

'■ 1 desire to call special attention to i^iv^. the only one of rav. 
lisi, by the way, whith is not|jven on Mr, Lodcyer's. This line, whicb 
was conspicuous at the ec%sc of 1869. scimis to be a/vayt pretmt 
in the spectrum of the chromosphere, and shows the fomi of it& upper 
surface or of a proiuberance nearly as weU. though of course not so 
lirighily. as the 3;9(i IiiiL\ It has do corresponding dark line in ibe 
r spectniiii, and not improbably may be due to the same 



LIST OF CHROMOSPHERIC LINES. 



603 



line was sufficiently developed for observation only along the edge, 
and at one or two bright points in the prominence, most brilliantly 
neither at its summit nor its base. Fig. 1 74 represents the appearance 
(the slit was perpendicular to the sun's limb). The case was similar 
with the magnesium lines." ^ 

Captain Herschel, in May 1869, communicated to the 
Royal Society a list of lines.* 

Subsequently Young has continued the special study of 
this subject with the most magnificent success, and has 
published a first list of 103 lines, which has been followed 
by a second of no less than 273. He also endorses my 
observation, that at times every line appeared reversed. 
The preliminary catalogue sufficiently indicates the names 
of the other observers who have studied the question 
by the initials affixed to various lines showing by whom 
they were first observed. 

The following is Professor Young's first catalogue,' 
obtained by observations made in September 1871. It 
includes only those lines which have been seen twice 
at least. 



NOTE H. 



1 



s. 

1 


1 

KirchhoflT. ' 

1 


• 

E 
•2 

& 
•5 


Relative 
Frequency. 


Relative 
Brightness. 


Chemical 
Element.- 


Previous 
Observer. 


I 


534 5 


7060-? 


60 


3 


^ 




2 


6545 


6677? 


8 


4 




L. 


3 


C 


6561 -8 


100 


100 


H. 


L.J 


4 


7190 


64957 


2 


2 


Ba. 




5 


7340 


64545 


2 


3 






6 


743*? 


643* • 


2 


2 






7 


768? 


6370- 


2 


2 






8 


8i6-8 


6260 3 


1 


I 


Ti. 




9 


8200 


6253 -2 


I 


2 ! 


Fc. 





1 Communicated to the Journal of the Franklin Institute^ October 
3rd, 1870 ; Nature J yo\, iii. p. 112. See also Journal FJ,, No. 524, 
p. 141, Sep. 1869. 

« Proc, R.S., vol. xvii. p. 507. 

' Reprinted from the Ametuan Journal 0/ Science and Arts^ stnd 
JVa/ure, vol. iii. p. 1 11. 



^^^iMH^^^^^^^H 


^""^^^^^^^ 


604 


SOZAK PuysKs. 




1 




NOTE H. 




1 








.1 


« 


ij 


IJ 


! 


fiuo'S 


6 , 8 


&. 


L 


IL 1), 


Ip3l 






Km. 


L 
L 




5871- 


100 






I.' 


1- ',':l\i 


5534 -o 

55*6-0 






Ff. 


R 




5454 S 


1 




Vt, 






S44S'9 






Ff. Ti 




_is iji.^vi 


S43Jt> 






Ft 






S«oo 










^1 fj7S-q? 


B- 






TL? 


: 




SVI" 






FftTL 






SJiifJ 






Fc 




-=5 M^>-5 


iJJ"4 






Pa 




-'■' l-i.! 1 -.1 


5360-6 








1 


-r '-I54 7 


JBio 






71 




;>1 14..jg 


g;:.' 






Ft 
Ft 




1465 ,.> 


Sja'' 










■ 





L/ST OF CHROAfOSPHER/C LINES. 


605 
















i 1 

1 ' 1 


Relative 
Frequency. 


Relative 
Brightness. 


1 Chemical 
1 Element. 


NOTE H. 


6 


Previous 
Observer. 


57 


1870-3 


5015? 


2 


2 


R. 


58 


1989-5 


4933 4 


8 


5 


Ba. 


L. 


1^ 


2001-5 


4923-2 


5 


3 


Fc. 


; R. L. 


60 


2003 2 


4921-3 


I 


I 






61 


2007 1 


4918*1 


3 


3 




L. 


62 


203 10 


4899*3 


6 


4 


Ba. 


L. 


63 


20515 


4882*5 


2 


2 




L. 


64 . 


F. 


48606 


100 


75 


H. 


J. L. 


65 


23585 


4629*0 




1 


Ti. 




66 


24193 


4583-5 




I 






67 


24355 


4571-4 




I 


Li. 


- 


68 


24440 


45646 




I 






69 


2446*6 


4563-1 




2 


Ti. 




70 


24578 


4555 




I 


li. 




71 


2461-2 


4553*3 


3 


3 


Ba. 




72 


24677 


45487 




>» 
J 


Ti. 




73 


24868 


4535-2 




I 


Ti. Cii. ? 




74 


24895 


4533*2 




I 


Fc. 




75 


24906 


453' 7 




I 


Ti. 




76 


2502*5 


4524-2 


2 


2 


Ba. 




77 


2505 -8 


4522*1 




2 


Ti. 




78 


2537-3 


4500*4 




3 


Ti. 




79 


2553? 


4491*0? 




I 


Mn. ? 




80 


2555'? 


4489-5? 


I 


I 


Mn. ? 




81 


2566*5 


4480-4 




2 


Mk'. 


L. 


82 


2581 5? 


4471-4 


75 


8 


A l)an»l rath 
tlian a lino 




83 


2585-5 


4468-6 




I 


Ti. 




84 


2625 


44430 


I 


1 


Ti. 




85 


2670*0 


4414-6 




I 


Fc. Mn. 




86 


26867 


4404*3 




2 


Fc. 




87 


2705*0 


4393*5 


3 


2 


Ti. 




88 


2719-? 


4384*8 




I 


C'a. ? 




89 


2721*2 


4382-7 




2 


Fe. 




90 


2734'? 


4372- 




1 






91 


2737- ? 


4369*3? 




I 


Cr. 




92 


2775*8 


43520 




I 


Fc. Cr. 




93 


2796*0 


4340*0 


100 


50 


H. 


L.J. 


94 


G. 


43070 




2 


re. 1 1. Ca. 




95 


2870 


43000 




I 


II. 




96 




4297-5 




I 


Ti. Ca. 




97 




4289*0 




2 


Cr. 




98 




4274-5 




2 


Cr. 




99 




42600 




I 


Fc. 




100 




4245-2 




I 


Fc. 




lOI 




4226*5 


1 


I 


Ca. 




102 




4215-5 




2 


Fe. Ca. 




103 


A. 4101 2 


100 


20 

1 


II. 


R. L. 



LIST OF CHROMOSPHERIC LINES, 



borj 



properly belong. He considers it to be probable that both 
these lines are due to the same substance which causes the 
D3 line. 

He also points out that the presence of titanium vapour 
in the prominences and chromosphere comes out very 
clearly from the catalogue, as no less than 20 of the whole 
103 lines are due to this metal. 

Young's second list,^ obtained by observations at an 
elevation on the Rocky Mountains, is preceded by the 

following: — 

Table showing the Number of Coincidences between the Bright 
Lines observed in the Spectrum of the .Chromosphere, and those of 
the Spectra of the Chemical Elements. 



NOTE H. 






Fe. Ti. S(,) 
Ba. S(^) 
S(w) Zn{^) 
Co. Ce. 

.. Ni. E(J 
Ca. Cr. Ce. 
,, Li. Zn. 
Ti. Ba. S(^) 
Ba. La. E(^) 
Fe. Ca. 

Ti. 

Mn. 

Cr. 

Ni. 

Ba. 

Zn. 

E(,) 



»» 
»» 
»» 
ft 
t» 



»» 



ff* 



*f 



f f 



• • 



t» 



ft 



Ce. 
Co. 

Mg. 

Na. 

S(w' 

La. 



10 

9 
4 
3 

3 

2 

2 
2 



Ti. 



»» 



t> 



»» 



> * 



»* 



»f 



S(.) 
Ca. 
Mn 
Ce. 
Sr. 
„ Zn. 
Ca. Cd. 
Ce. 
Co. 
Cr. 
Sr. 



»» 



3 
2 

I 

I 

I 

I 

I 

1 

I 

I 

1 



S( J E(J 

Mn. Zn. 

Cr. E(^) 

Ce. Co. 

Na. Cu 

lines marked 
with an * 



Unknown. 

Ke. 

Ti. 

Ca. 

Ba. 

S(J 

Mn. 

Ce. 

H. 

Na. 

Cr. 

Mg. 

Sr. 

Zn. 

Ni. 
Co. 
Cu. 
La. 
Ru. Ir. 
Cd. 
Li. 



52 Total. 



64 

23 
10 

8 

7 
6 

5 
4 
4 
4 
3 
3 

3 

2 

2 
I 
I 
I 
I 



no 

43 
29 

«3 

14 
12 

II 

4 
6 

10 

4 
6 

9 

9 
6 

5 

2 

3 
I 

I 

I 



H 



The numbers in the last column denote the whole number of times that the 
symbol of each element appears in the catalogue, either singly or combined 
irith others. 

He states : — 

** The great altitude of the station (nearly 8,300 feet), and the con- 
sequent atmospheric conditions, were attended with even greater 

* Nature^ vol. vii. p. 17. 



a (i vantage 5 
althoiis;h I v 
FiTiunli lifer 



SOLAR PHYSiCS. 



special worit Huin had been really eKpentd. 
IT quite able to realiM! my bi>pf of s^ing tX\ iht 
. i.T-.ed, unless once or twice for a monicnt, dLiing 
It.inces of the solar surface. 
m , lioweTer, mnlirmod my belief ihal the v. 
■ LI the base of ihc chromospheic. and thai 3 
ill roversed at aoy moment depends rocrtly u 
.ind atmospheric conditions." 

Uright Lines \vi i 



e uriri^ 
ihaiA 



7»SS-! 


I«S 


„ 


Si? 


as 


SO 


lOO 






:i 




■o 


; 


smu 


30 






5 






5 




63,=-6 






'373- > 


5 
■: 









LIST OF CHROMOSPHERIC UNES. 609 


• 


P.C. 

i3t 






K. 


B. 






K. 


A. 


NOTK n. 


No , 


E. 


41: i 


D,ioi6s» 


58749 


, 100 


90 




42 




1031-8 


58527 


i 8 


2 


Ba. 


43 ' 




"351 


5708-3 


I 


I 


Fc 


44 




1151 I 


56872 


2 


« 


Na. 


45 




1154-2 


5683-5 


5 


3 


1 . . _— 


46 




1155*8 


5681-5 


2 


I 


: Na. Fe. N(,) 


47 




1165-7 


5667-8 


2 


2 


S(.) 


48 




1167-0 


5666-0 


I 


I 




49 




1170-6 


5661-5 


15 


2 


Fe. Ti. E(,) 


50 . 




1 1750 


5656-7 


8 


3 


; S(,)N(.) 


5? 1 




1176-6 


5654-4 


2 


I 


Ke. 


52 . 




1187-1 


5640-2 


' 


I 


' S(.) 


53 ; 




1189-3 


5637-3 


I 


I 


1 


54 ; 




12006 


5623-2 


2 


I 


' Fe. 


I 55 ' 




1207-3 


5614-5 


2 


I 


Fc. 


56 1 




1229-6 


5587-6 


2 


2 


Ca. 

1 


57 1 




1231-3 


55855 


2 


I 


! Fe. 


58 


I4t 


12742 


55341 


50 


12 


Ba. Fe. Sr. 


59 , 


•S 


1 281 3 


5525-9 


40 


5 


Fe. 


60 




1287-5 


5518-7 


15 


2 


Ba. 


61 




1 298 '9 


5505-8 


2 


I 


Fe. 


62 




13035 


5500-5 


2 


I 


Fe. La. 


63 




1306-7 


5496-6 


2 


I 


Fe. E(,) 


64 




13206 


54802 


2 


I 


1 Ti. Sr. 


65 




1324-8 


5475*9 


I 


I 


Ni. 


66 




13287 


5472-3 


3 


I 


t 

1 


67 




13370 


54623 


I 


I 


Fe. N(,) 


68 


16 


>343 5 


5454-7 


10 


4 


Fe. 


69 


17 


>35>> 


5445*9 


10 


4 


Fe. Ti. Br(^) 


70 




1360-9 


5435 '4 


5 


2 


Zn Br(,) 


7" 




13629 


54330 


2 


2 


Fc. 


72: 


18 


1364-3 


5431-8 


8 


5 




73 


19 


13670 


5428-8 


8 


3 


Fe.Ti. 


74 


20 


1372-1 


54245 


25 


6 


Ba. Ti. S(^) 


75 


21 


1377-4 


5417-9 


5 


2 


Ti. Mn. 


76 




1380-5 


5414-5 


2 


2 


Fe. 


77 


22 


13825 


5412-4 


4 


2 


Mn(,) 


78 




13847 


5410 


2 


I 


Fe. Ni. 


79 




13857 


540; 


2 


2 


Cr. 


80 




1389-4 


5404-8 


2 


I 


Fe. 


81 


23 


1390-9 


5403-" 


5 


3 


Fe. Ti. 


82 




1394-2 


5399-6 


2 


I 


1 Mn. 


83 


24 


13975 


5396-1 


4 


2 


' Fe. 1 i. 


84 




1401 -6 


5392-2 


2 


I 


1 Fc. Ce. 


85 




1412-5 


5380-2 


3 


2 


Ti. 


86 


25+ 


1421-5 


5370-5 


10 


3 


Fe. 


87 




1423-0 


5367-0 


I 


I 


Fe. 


88 




1 ^^'^V^ 


53665 


I 


I 


Fe. 


89 




1 1428-2 


53640 


I 


I 


Fe. 


90 


26t 


1430-1 


5361-9 


20 ' 


10 


Fe. 


9' 




1438-9 


5352-4 


4 


2 


Fe. Co. Ce. 


92 




14467 


53450 


I 


1 




93 




14508 


5340-2 


1 


1 


Fc. Mn. O(^) 
K K 




loot 

lOI 


31+ 


'03 


3* 


104 


I 33t 


l'^ 


34 




35 


109 


36 1- 



'589- 

'590' 

'597 ■; 



16113 
1613-9 
16156 

16. 7 '4 
l6i8'9 
16371 
16181 
1631 5 



LIST OF CHROMOSPHERIC LINES, 



No. ' PC. 



K. 



A. 



'47 




1701-8 


51330 


148 




17047 


5130-8 


149 




1707-9 


5128 6 


150 




17107 


5126-7 


»5i 




1712-2 


5'25-5 i 


152: 




i7'3'4 


5124-4 


'53 




1715-2 


5123-2 


'54 




17179 


51210 


'55 




1719-4 


5"9 9 


156: 




'7273 


1 5 "49 


'57 




'7346 


' 5108-8 


158 




'7377 


5107-0 


159 




17504 


5098-1 


160 




1752-8 


5096-5 


161 




1 765-0* 


50870 


162 




'771-5 


5083-5 


'63 




'778-5 


50779 


164 




1823-6 


5047-8 


165 


1 


'8334 


5041-2 


166 




1834-3 


5040-1 


167 




18489 


5030-1 


168 




1856-9 


1 5023-5 


169 


56t 


1867-1 


5017-6 


'70 


57+ 


1870-6 


, 50150 


'7' 




19051 


4993*3 


'72: 




€.1961-0 


i 4956-7 


'73 


58 1 


'9895 


4933*4 


'74 


59+ 


200 r6 


4923*' 


'75 


6o^ 


2003 2 


49213 


176 


61 


2007 2 


49182 


'77 




2016-0 


4911-2 


178 


62 1 


2031-1 


48993 


179 


^1+ 


2052-5?* 


4882-9 


180 




2067-8 


4869 4 


181 


64t 


V 20800 


48606 


182 




20876 


4854-7 


'83 




20940 


4848 I 


184 




2ii6-?» 


4826-5 


185 




2121-2 


4S228 


186 




2142-4 


48044 


187 




2171-5 


47787 


188 




2229-1 


47308 


189: 


1 


2251-3* 


4712-5 


190 




2309-5 ' 


4666-3 


19' 




23 '43 ' 


46633 


192 




23230 


46560 


193 


65 


2358-4 


4629 


194 




2359*5* ! 


4628-2 


195 




2369-7 


4620-3 


196 




2410-2 1 


45894 


197 




2412-8 I 


4587*5 


198 


66 


2419-3 


45832 


199 




2429-5 


4576*0 



2 

2 

2 

4 
3 
30 
30 
2 
I 

30 
40 

30 
20 

3 

30 
10 

5 
100 

5 

3 
I 

10 

3 

3 
I 

2 

3 

2 

2 

'5 

2 

I 
I 

2 

'5 
4 



LINES, 




611 


— 


E. 


NOTK H 


B. 1 





2 
2 
2 
2 

3 
I 

'5 
10 

I 

2 

8 

12 

8 

3 

2 

6 

4 
I 
80 
2 
2 
I 
2 
I 
2 
I 
2 
I 
I 
I 
8 
I 
I 
I 
2 
6 
2 



Fe. 

Fe. 

Ti. 

Fe. Ti. 

Fe. 
Fe. 
Fe. 
Ti, 

Ni. 

Ti(J 

Fe, 

Fe. 

Fe. S(^) 

E(.) 

Zn(J 

Fe. 

Fe.? Zn(^) 

Pe. Ca. 

Fe. 

S(J 

I Fe. Ni, 
I Ti(J 

Fe. N{J 
I Fe. 
Ha. 
Fe. S(^) Zn( J 

Fe. 

Zn(^) 

Ba. La. E(^) 

Ce. 

II. 

I*e. Ni. E(^) 
Ca. 0(J 

Mn. 

Co. N(J 

Fe. 

Ce. 0(J 

Fe. Ti. 

Ti. 

Ti. N(J 

Cr. 



K I< 2 




ao? 


' 


246] 


»S 


73 


246; 


209 




24S0 




73 


24S6 


Vil 


74 


2489 
2490 


ai3 


76 


2502 


Jt4 


77 


2505' 


1:1 




3517 




2518-. 


ai7 




2S27X 


2t8 


78 


»537" 


219 


79 


2552-. 




80 
Si 


\\^\ 


JMI 


82+ 


i2\%vi 


113 


83 


a 


324 




aas 


84 


2625-2 


226 




26330 


227 




2639-6 


23$ 




26515 


229 




26532 


230 




26649 


23" 




2665-9 


233 


8S 


2670-0 


233 




2680-0 


, 134 




2686-8 


13s 




2696 B 


236 




2698-2 


237 


87 


2702 -s 


=38 
239 


8S 


27(52 

3718-5 


240 




2720-3 


241 


89 


2721-6 


141 




2735 -8 
27*8-0 


243 

0^ 


. 



UST OF CHROMOSPHERIC LIXES. 



613 



No. 


P.C. 


K. 


A. 


F. 


B. 


E. 


253 




28430 


1 4313*5 


I 


1 


TL 


254 


94 


0.2854-2 


4307-2 


3 


1 2 


Ca. Fc. 


255 


95 


28677 


4302*1 


3 


2 


Ca. Fe. 


256 


96 


2874*2 


4298-0 


I 


I 


Ca. Fc. 


257 


97 


28945 


42894 


I 


I 


Cr. Ca. Ce(^) 


258 


98 


29285 


4274-6 


2 


I 


Cr. Ca. 


259 


99 


8961-2 


4260-0 


2 


I 


Fe. 


260 


100 


2996-2 


42452 


30 


3 


Fe. 


261 




3018*0 


42355 


30 


5 


Fc. 


262 




3022-8 


42330 


15 


5 


Fe. Ca. 


263 


lOI 


30400 


42263 


3 


3 


Ca. Sr. 


264 


K)2 


3061-8 


^^i'l 


40 


7 


Ca. Sr. 


265 




31555 


4178-8 


I 


I 




266 




31870 


4166-7 


I 


I 


Ca. 


267 


io3t 


h. 3363 '5 


4101-2 


100 


50 


H. 


268 




343 »o 


40770 


25 


2 


Ca. 


269 




35260 


40450 


3 


2 


Fe. 


270 




37033 


3990? 


2 


I 




271 




37695 


3970? 


2 


I 


Fe. 


272? 




Ht3778-5 


39679 


75 


3 


Fe. Ca. 


273 




H, 3882 5 


3932-8 


50 


I 


Fe. Ca. 



NOTE H. 



Notes. 

1. The position assigned to this line, first observed by Respighi (a fact of 
which I was ignorant when the Preliminary Catalogue was published), rests 
upon two series of micrometric measurements, referring it to four neighbouring 
dark lines — the probable error is about ^V^^ o^ * division of KirchhofT's scale. 

9. No. 6 in P.C. Position there given, 743? 

16 and 17. Nos. 8 and 9 of P.C. Position given as 816 "8 and 827-6, by a 
mistake in identifying lines upon the map. 

40. I have never myself seen this line reversed. Prof. Emerson, however, 
saw it several times. It was first reported by Rev. S. J. Perry, in uVa/ure, 
vol. iii. p. 67. 

41. The position of this line has been independently determined by three 
scries of micrometric comparisons with neighbouring lines. My result agrees 
exactly with that of Huggins. 

72. Erroneously given in P.C. as 1363*1, which line does not reverse, or at 
least was never seen reversed at Sherman. 

100. The principal line in the spectrum of the corona. The corresponding 
line in the spectrum of iron is feeble, and on several occasions when the neigh- 
bouring lines of iron (1463, &c.) have bten greatly disturbed, this has wholly 
failed to sympathise. Hence I have marked the Fe with a ?. Watts indicates 
a strong line of oxygen at 53 1 5 A. 

152 and 156. Ob^rved only on one day, but verified by Prof. Emerson. 

172. Called ** little C " by Mr. Stoney. 

179. Given by Lockyer as K 2054. Its position is a little uncertain ; it 
seems to coincide with neither of the dark lines at 2051 nnd 2054, but lies 
between them, a little nearer to 2051. 

189. Rather a hzn(\ than a line. 

222. The position of this line, which, however, like 189, is lathcr m band. 




" In the catalogue, 
numlM^r : a { refers t( 

" The numbers in 
Catalogue,' containing 
the Ameruan JoumA 
that some other observi 
publication of the lin 
abnost solely upon the 
Royal Society (which 
Kayet, and Secchi), it ii 
be marked in the same '- 

" The third column, d 
Kirchhoff's scale, the nJ 
continuation of Kirchhod 
that the map shows nol 
pnsiiion, not the CKisted 
uncertain. I 

*' The fourth column, h 
icn-millionihs of a millim 

"'The numbers in this 
from the maps themsel 
account of the shrinking 
lion of printing, but from 
Angstrom which accomp 
nary Catalogue the numb 
slight discrepancies in th 

" The fifth column, ma 
centage of frequency wit 
weeks of observation ; ai 
their maximum brighines 

" The variations of brjl 
much disturbed, were so 
great weight can be assig 
inferred ihai lines which 



LIST OF CHROMOSrHERIC LIXKS. (,t ;j 



one time and another, but not seen steadily enough or long enough to n«ii i^ ii 
admit of satisfactor\- determination. 

** The last column of the catalogue contains the symbols of the 
chemical elements corresponding to the respective lines. /Ihc ma- 
terials at my disposal are the maps of KirchhotV and Angstrom, 
Thaldn's map of the portion of the solar spectnim above (I. and 
Watts' •* Index of Spectra." Since the positions , of the lines in the 
latter work are given only to the nearest unit of * Angstrom's scale, * 
I have marked the coincidences indicated by it with a (w), considering 
them less certain than those showD by the maps." 

Professor Young thus sums up his work: — 

" In addition to the elements before demonstrated to exist in flu^ 
chromosphere, the following seem to be pretty positively indicated 
sulphur, cer.um, and strontium ; and the following with a somewhat 
less degree of probability, zinc, erbium and yttrium, lanthanum and 
didymium. There are some coincidences also with the spectra of 
OX) gen, nitrogen, and bromine, but not enough, considering iIm* total 
number of lines in the spectra of these elements, or of a r liara^ h-r, ti 
warrant «iny conclusion. One line points to the presence of iiidiuni 
or ruthenium, and only three are known in the whole spectrum of 
these metals. 

** No one, of course, can fail to be struck with the number of ravs» 
in which lines have associated with them the symbols of two oi mor*- 
elements. The coincidences are too many and too closer lo b*- all tl.<- 
result of accident, as for instance in the case of iron and i.uU utiu, ni 
iron and titanium. 

"Two explanations suggest themselves. Tlu- lir^i, wliMh svmo* 
rather the most probable, is that the metals opirai^d upon )// il»<- 
observer who mapped their spectra were not ab-.o|iii<l> pni' miKm 
the iron contained traces of calcium and titanium, or ////• /'//»// If 
this supposition is excluded, then we seem to \h; tltmu lo ili*- * on* l<i 
sion that there is some such similarity Ixrtwern th<- mol* mj1« m of M#« 
diflcrcnt metals as renders them susceptible- of /iiiam ^/i»' Ioo^'/h- 
periods of vibrations— a resemblanre. as ny^nrtl'^ On- tn^thtt* t tn 'An* \t 
the molecules are built up out of the const ifu«fii aioms, s«ff«' <« "' ''/ 
establish between them an important physical 'and |/i//Im)/J/ * Iw hi*' *h 
relationship. 

** I have prefixed to the catalogue a tabl'* ^\i'tv,ttty^ if** tiuini** t of 
lines of each substance, or combination of ^ub^^.^n• < 4, ol/".< t '»*\ t** 'f"- 
chromosphere spectrum; omitting. how*r/< ;, o//;f« i», tn*tff/*h, •*''( 
bromine, since with one exception 'lifiT 2'y*^ m iO»m of *)»**" * '* * 
stands alone, or accounts for any lin* -^ i»o» o'fx tv,.j * / j/).*.*.' »\ 

Quite recently Profcs.v^r Vounj^ li;j ; Ji'yv/n iIh j/'/ il/')»*y 
of distinguishing between tJiov; \\u* >. v/hi' U **» ■/)"'/ 
reversed in the chromosph':re hu*\ iIj'/v f,\,,* h, * '.ttnn,/ it'tot 
greater depths, are thrown into it lyy ;;/»..),' 

" 1 must po» ^io■-' y.,*:,fy:* ; '. • ;' •'/ •/.'. i »,-, •, t • » *. > 
enable u:> to d;*'*: ;j j.^:<. ♦'/ .'/;:,' «/":.* V"'-* < '*' ■/ •-»'* 



LIST OF CHROMOSPHERIC LINES, 617 



" In the artificial eclipses produced by our spectroscopes, this complex no ie h. 
constitution of the chromosphere is only obser\ed in some almost — . — '- — ' 
exceptional cases, and in a ver>' imperfect manner, while \n general 
we cannot obtain anything but the reversal of the lines of hydrogen 
and of the line D3, which is held by many a§ not belonging to that 
substance. 

" This depends on the slight thickness of these gaseous strata, by 
reison of which they are always veiled and concealed in the undula- 
tion of the limb, and thus submerged in a sea of vivid solar light : 
which does not happen in real eclipses, in which the disc of the moon, 
covering the sun before the luminous rays can arrive within our 
atmosphere, renders the undulations, which in full sunlight invade the 
slit of the spectroscope, impossible. 

" But even in full sunlight, favourable conditions are at times pre- 
sented for observing these bright lines, at least in the case of those 
substances which, after hydrogen, are most elevated in the photosphere ; 
and this is especially the case in the regions of the spots and of the 
dark granulations. Indeed, in these localities, I have often been able 
to observe at the base of the chromosphere, through long tracts 
of the limb, bright lines besides those of hydrogen, the two red 
lines between a, B, and B, C, almost always, and very marked. If 
I have generally confined myself to the observation of these last lines, 
I was obliged to do so in order not to distract myself by too many 
operations while 1 was already sufficiently occupied in observing the 
solar outline, and in the study of the protuberances. These regions 
I believe to be those in which Tacchini finds the lines of magnesium 
reversed. Indeed, comparing the regions in which Tacchini finds 
these magnesium lines, they appear to me generally coincident with 
those in which I see the two bright lines between a, B, and between 
B, C ; with this difference only, that these last lines are extended 
to a smaller part of the outline. But this might depend on the 
greater difficulty of observing them, because projected upon a bright 
field, while those of magnesium are more easily rendered visible 
through the striking contrast of their corresponding black lines. And 
this supposition is, I think, confirmed by the fact, that the two lines 
between a, B, and between B, C, are those which, after the lines of 
hydrogen are most prominent, not only at the base, but also in the 
most elevated parts of the protuberances ; a manifest proof of the 
great predominance of that substance, from which proceed the lines 
in question, in the composition of the mass and of the superficial 
stratum of the sun. 

" The visibility of these bright lines in the regions of the spots and 
of the granulations might be explained by the greater elevation of 
the gaseous strata, caused by the prevalence of jets or eruptions, 
generally composed of several substances besides hydrogen ; but with- 
out excluding that explanation, 1 maintain that another fact may 
contribute to render these lines more easily seen. 

" It is a fact that with the slit arranged tangentially in the locality 
of the spots and of the granulations, the spectrum is furrowed or 
divided by lines almost permanently dark ; and this is, I maintain, 
produced by the dark or less bright streaks of the solar limb under- 
neath the chromosphere, on which the undulations of the outline 



refractor (similar to t 

useful contrivances ba 
oljji'Ct-filass for the (I 
ihe almost continuo 
refrangible brighl li 
from the equator to 7; 
these observations du 
portant that they shou 
also in the most norma 
solar activiiv, the visit 
of the lone indicated 
iinre of the large proi 
or characteristic of the 
nearer 10 the equator."' 



" The regions nf the a 
and III,, when theyarel{ 
some special characicri 
protuberances or jets \ 
chromosphere in those n 
ami solid, and in condili 
hydrogen and D, bright! 
.imnng which are sodiui 
red lines, one between B 
nearly correspond to tl 
Kirchhoff. 

" The substance which 
dominant in these jets, i: 
lines between iiH and b 
constantly observed thesi 
protuberances near to tt 
and follow them even in 
far as 2". from the limb ; 
spectroscope I have obs 



LIST OF CHROMOSPHERIC USES, 619 



protuberances : the lines of sr^iium only have been sometimes obser\'ed notk 11. 
at some distance from the limb. 

" As regards those jets which are compound, their general and funda- 
mental trait is that thev contain hvdro^en from which the other 
substances seem to be carried off to a greater or less height, only 
in some especial ramifications ; so that the hydrogen must be con- 
sidered as the fundamental and predominant element of all the solar 
eruptions. In the ver\- numerous and ver\- accurate obser\ations 
made by me of the eruptions near to the sfX)ts, I have been unable to 
observe jets of any substance not accompanied by hydrogen. 

** In my Note III., in treating of the protuberances with complex 
spectra, 1 remarked that the two red lines between aV» and between BC 
often appeared ver\' vivid in some parts, and especially in the 
summit of some jets, while they were totally wanting in the lower 
portions, so that the gas corresponding to these lines constituted true 
clouds enveloped in hydrogen. This fact I had previously verified 
also for magnesium, of which I had often seen clouds mixed with hydro- 
gen to a remarkable height from the solar limb, almost as high as T. 

** This fact seems to me coincident with \\ at of the nodes or nuclei 
found bv Profcs.sor Lorenzoni in his excecdir.Lflv accurate observations ; 
he observed these cften in the protubeiances with complex spectra ; 
distinguished from the other parts of the protuberances themselves 
by the multiplicity of the spectral lines, namely. yi b, I), &c. Generally 
it is ascertained that the visibility of these lines in the elevated 
parts of the jets is of brief duration ; which would prove that the 
substances to which they belong constitute jets or intemiitting eruptions 
of short duration, returning rapidly to the surface of the sun, or 
rapidly losing the vividness of their brilliancy. 

** In the jets near to the spots, or rather, in those points or bright 
solid masses which arc often observed in proximity, and sometimes in 
contact with the nuclei, the spKrctrum is generally composed of very 
numerous bright lines, of which I have often ver\' distinctly seen as 
many as twenty, although I was not expressly engaged at the time in 
the observation of them. On some rare occasions, the whole sjxrctrum 
appeared momentarily reversed or luminous beneath these masses 
of bright points, and sometimes 1 remarked a fact, which seems 
to me worthy of notice ; namely, that the spectrum became luminous 
in some portions only, which beginning at the lines C. Do. and F, 
extended to some distance towards the least refrangible parts of the 
spectrum, gradually diminishing in brilliancy."* 

Father Tacchini has observed the following bright lines 
in the spectrum of a prominence: — Hydrogen, D.t, H,C — 
B, a, *i, b.^, A,. *4, 4943 (double), 5031 (double), 519.^. 5229. 
5272, 5282, 5265, 5316, and two other undetermined lines 
between b and 5316.^ 

Professor H. C. Vogel has observed the following bright 

' Kcspi^hi, Note v. § ix. 

- Lviiiptcs Rii'ntu^, vol. Iwvi. p. 829, March 31, 1873. 





The magoesiin 
"■ere not se«n.t 

Professor Lo^j— 
frequency rf,pp,~; 
Of t.«i<}-si, p„^^ 



THE LINE 1474 {KIRCHHOFf), 621 

NOTE I.' 
The Line 1474 (Kirchhoff). 

This line was first observed by me in the chromosphere note i. 
on June 6, 1869 (p. 496). It was then seen by the Ameri- \ 

can astronomers in the corona in the eclipse of that year. 
My most recent work with regard to it, showing that it 
cannot be an Iron line, will be found referred to at p. 553. 

Professor Young's and my own early opinions as to the 
origin of this line will be seen by reference to pp. 268 and 
271. 

Secchi, in 1869, suggested that an investigation of the 
hydrogen spectrum would throw light on the subject. 

He says : — 

" Uhydrog^ne nous a donnd, dans la boule du tube du p61e positif, 
une raie assez belle, placde presque k ^ale distance des deux raies C 
et F (a et /3) du spectre solaire, mais un peu plus rapprochde de F : or 
cette raie coinciderait avec celle que Nl. Young a obser\'<5e dans la 
demi^re dclipse, comme propre k la couronne. On Ta attribute au fer, 
ct j'ai d*abord suppose que, dans mes tubes, elle pouvait proven! r des 
r^phores mdtalliques ; mais les tableaux que je poss^e ne m'autorisent 
pas k admettre cette conclusion ; je I'ai vue d^ailleurs dgalement dans 
des tubes de Geisslcr, qui sont tr^s-purs et tr6s-peu fluorescent s ; je 
crois done qu'clle appartient rdellement aussi h Thydrog^nc, mais 
qu*elle se d^veloppe sous une temperature plus basse, car je I'ai vue 
seulement dans la boule qui environne le rdophore." * 

Professor Young has been independently led to consider 
this line as probably not due to iron, from the fact of its 
want of sympathy with other undoubted iron lines : — 

" The corresponding line in the spectrum of iron is feeble, and on 
several occasions, when the neighbouring lines of iron 1463 have been 
greatly disturbed, this has wholly failed to sympathize ; hence I have 
marked the Fe with a ? "' 

In the same year. Professor H. C. Vogel remarks, when 
dealing with the chromospheric lines, ** 5315*5 eiscn .^ 
Schien nicht genau zu coincidiren, die eiscnlinie hat die 
Wellenlange 5315*9:*'* thus indicating a difference of 

* See p. 496. 

' Comptes Rettdus, vol. Ixx. p. 84, January loth, 1870. 

' American Jottrnal of Sciences and Arts, vol. iv., November 1872. 

* H. C. \'ogcl, Beobaciit unseen, &c., 1872. 




*^!^ Sooo after 1 
promtnences tn 
an eclipse, a ft, 
sun enabled aic 
of its contour. 

In the same 
sent a note to ta 
folloH-ing announ. 
November. s86S t 
the 25th of that'n 

I'Maisceouonn'ai 

vo,s.nagedulx.rd«,lJ 

» raie noire disparaf, , 

un effet de conirasie 

renverscment est seule 

r el dans plusieun aut 

On the 1st March 

to the Academy oJ 

the following words 

"Ayant agrandi nota 

f>oleiI qu, tombe sur la 

j'per9uque,enj-faisfl„ij 

'cs pfotuWrances M la 



THE COSTIXUOCS SPECTRUM STRATUM, 623 



rose nest pas en eontaet eon t inn avec Ic bord solairi^ He noik k. 
states that this is only seen in perfection with a magnified 



image. 



He then proceeds to examine his observations in the 
following way : — 

" A ma gninde surprise, j'ai vu alors disparaitre compl^tcment toutes 
les rales fines, ct les raies noires D et ^ restait h peine visibles. J'ai 
d'abord cru que c'etait W un effet de la faiblesse de la lumiere, mais j'ai 
v(5rifid qu'il n'en etait pas ainsi, car je pouvais voir tr^s-bien les raies 
au dehors de la couche rose, et imm^iatement sur le bord du Soleil. 
J*ai attribu^ Teffet obser\<5 k la faiblesse du spectroscope, et je Tai 
avait montd avec trois prismes tr6s-dispersifs : Ic rdsultat a ct«5 le meme. 
On pourrait supposer que le phdnomene est du h un effet de I'agitation 
de I'air, pros du bord solaire, mais cette hypothese elle-meme m'a pani 
devoir etre exclue par cette remarque que je distnguais tr^s-bien les 
raies brillantes de la couche rose au milieu de I'oscillation la plus 
vive : les raies fines obscures sont alors visibles dans tous leurs details. 
11 me parait done que nous sommes en face d'une couche qui donne 
rdellcment un spectre continu." 1 

He then adds that this stratum is difficult to observe, on 
account of its extreme thinness. 

Further on still, he says — " Cette eonche, proportionelle- 
vient assez mineCy serait eelle on a lieu le renversement selon 
la theorie de Kirehhoff ; " and then proceeds to argue, that 
though very thin, it is deep enough for the purpose. 

References to p. 497 will give my views of this matter, 
which are simply, first, that I have never seen an absolutely 
continuous spectrum in the place indicated by Secchi, 
though I have often seen nearly a continuous spectrum 
given by facuhr, an observation endorsed by Captain Hcr- 
schel.^ Secondly, that a considerable reduction in the 
number of absorption lines observed here is easily ac- 
counted for by the fact that two spectra are superposed : 
and thirdly, that I cannot admit that ''la eonchc rose 
nest pas en eontaet eon t inn avee le bord solaircy 

When Professor Young in the clear atmosphere of 
Mount Sherman saw many lines reversed, he considered it 
might be the stratum to which Secchi referred, but on 

> Comptis Ki'fi<fits,\o\. cit., p. 581. 
- Proc. R.S,^ vol. xviii. p. 64. 




ing that at the bai 

vapours present is 
are still as far off 
continuous spectr 
the sun. 

I here give the e 
to which I have all 

"At the very base of 
I* or is' from the edg 
dark lines which are nl 
vanish more or less coni 
of an old and sonie*hatl 
M the edge of the sun a 

"This is not strictly 
of the air is so much inc 
dark lines to vanish, a m 
can be little doubt that y 
removed, this lowest poi 
same spectrum of bngh 
end of totality during an 



The Class? 

As early as 1S69. : 
into two great groti 
April 27. iSro^hat 



THE CLASSIFICATION OF PROMIXEXCES. 625 



In the sitting of the Saxon Academy, on the 2nd cf 
June, in the same year, Zollner, in a paper laid beiore that 
body, announced that he had arrived at the same result : — 

" The forms of ih'e protuberances may be divided iriD: rxi :h.irii:- 
teristic groups— into the vapour or cloud-like, and :n:i: -jz^ tri^ri- -i 
formations. The preponderance of the one or the ':th.er r.pe izzfir^rz 
partly to be dependent on local condi:ior.5 on the surtica .1 die sir. 
partly on the time ; so that at particular periods the 're. it -dicr= zitt 
other type preponderates. The striking rescmbLiZite zt "±e :j:«ai-jt.» 
formations to terrestrial clouds is readily expiair-ed. •» b.:i-. t a z-r.r, tr!r".i 
that the forms of our clouds are due not to the z.ir::.:! zi .f vi-tr m- 
pended in them, but essentially to the nar-rt az-t T^irr.zr - ■» :. -^z 
the differently heated and agitated zias^er :t i.r ir- =^li.i .il:: r>-' 
particles of aqueous 'ip^ur ^rz\ ;- .vrr^;.--.-;. , . j .:. .-.wfj''- zu 
material by means of uhuh tke z'-r. --^iKZL-x^J zzj-. -.t^., >,-i-'-T Ut; 
masses of air are rentier-: J rziJer: :■ 'a:. 7"'.^ r - :"'"'-: y.izf.L. -^-rr 
masses of hydr'-^gen is the .'zu:: :/ :ki -:.:;:..:• - :•. -^ -a^, ' . 
protuberances." 

Young: at once accented thi: :lirr fiLi .1 — 

** About forty dirferer*: pr : — .-*-_:-:; -.i t :•=•- -— - — .-r . >. - 

fully obsened from Sep:=n":>tr :t :. '.-r.-.-j^r % .'-, - -: - 

been sketched. Moii of :r.=n: :'i- t-s.:._-s_ t^ _..: -.... _^ . -^ 
established by Zollner ar.d Li^ik". tr 

On the 4th December, \\'i 'T^i\y:'^ -tr^^ >^:' -* - .. 
Reale Accadcmia dei L:kw: hi- "Ji ri - .v. • : , 

contained the very Ciref-". ir.d t:.i-"^,-i -, 
given below :— 



< • . .". — . 



--4 



" In the immense varety ::' :': — : -.-: :tt t- ^ _ v-.-- ■ 
ing t>T)es are chiefly ti be rziz.-zx.'.i — 

'* I. Wcll-detined. d*.'.. litt l: 1 -. .L-.t-: t- . 

" II. Jets united m ^- -i = 

** 111. Jets with rarr.:f:ti.t t:.: i-.i 'i.-.u^ . .-.: 

'* IV. lets of larger ji r.. : :- :.,.-:-..»- - . - ; - . . 
lated. 

"V. Jets or cloudy ',',\.t:.z.\ .^.\'=i *- I'j,-,. - . . 
summit with cloud; arth-.:. 

"VI. Cloudy rr*i-;e5, .!Te^*-li^' v,-: v.^o'. "•-', • 

**VII. Mas^ri vr cl'-.-Ci &fr-^-_r>.-vt '-vv •-.■ .. - - 
"The dcliccV.'j ir.I » ■.;!!.• :,-,-: tr.i i -• . . - . - 
locality of the ^ji-:-. v.:.-:r-. :'-::..-:«' --.r- 
chromosphere :r. vt.-; ". rr: ■.i'rrsi':: 
inclined loAirda \\t v*:r..*:,t. i.-. • ::.'.-; '.-. 
summit: at tiTits r-.-.'.-i :rwLri: t •-<■-;•. '^.. , '"" 
the sun. 






*^ 



* Journahf the Fr-.'ifi'.m h. :l:^u, -.V:"^ 



i. M^* 



i 


and we often see some 

"Ofthisclassificatio 

that the fundamental 

1 that of well-defined! 
groups."' 

It Will be seen, 

my fundamental cj; 
I also had observ 
of themextendingc 
tlie polar regions. I 
Respiglii, who state 

"These gigantic dust! 
environs of tlie regions c 
of them near the poles 
nevertheless these prot 
duration than those whi. 
quickly developed and ■ 
previous Notes. Somei 
the higher latitudes alsi 
fainter in brightness and 


B 


■ Note jii. } 2, p. 9, 
' Note V. AttJ, p. 263. 
'Lately some observer 
tures, offering this dlstin 
brought to light by theii 
and pronouncing the sma 
ing of the fibrous stnicti 
observer had before succ 
necessary to «'aste any 1 



THE CLASSIFICA7I0S OF PROMINESCES. 627 



" Generally, these consist of jets or fine threads, either parallel, con- note i. 

verging, diverging, or curved at the summit, and often converging to a 

conical or pyramidal form. These jets or threads are occasionally cut 
short at the base, presenting the appearance of a shower of fire ; and 
in spite of the weakness or the slight intensity of their light, they are 
shown to be rather more consistent and enduring than the vaporous 
masses. Generally the large protuberances can be divided at their 
base into many distinct jets of various sizes and degrees of brightness, 
which at their summits are diffused in the most curious way, being 
blended in a mass of light, often terminated irregularly in feathers or 
ramifications either rectilinear or cur\ed. 

" Sometimes, however, we see in the vicinity of the spots, masses of 
hydrogen, solid at the base and cloudy at the summit, of moderate 
height and extent, and extending over several degrees of the solar 
limb. 

*' Also, in the isolated masses or clouds the dimensions are exceedingly 
variable, some of them being reduced to delicate threads, isolated or 
combined in groups ; others, on the contrar\', are vaporous masses, 
more or less solid, and more or less irregularly terminated, and of 
enormous volume." * 

In October 1871,^ Secchi sent to the Paris Academy a 
classification of his own. Of this I give an abstract ; it 
will be seen that it agrees exactly with the divisions of 
former observers, to whom, however, he makes no reference 
whatever. . 

He mentions the following classes : — A vtas, jets, panadus. 

The first he subdivides into amas brillants and amas 
cumuli for me 5, These two sub-classes include all the dense 
and massive prominences ; the first sub-class is considered 
as consisting of local upheavals of the chromosphere not 
higher than 15" or 20^ 

The second class, yV/j, includes all the narrow and pointed 
prominences. The meaning of the term is so well under- 
stood, that no further description is necessary. 

The third class, panaclus, includes all not compre- 
hended in the two former classes, such as all the jet-like 
prominences of considerable width ; the smoke-like and tuft- 
like masses ; those like those ; the interlacing filamentous ; 
and those which present that curious cellular structure so 
rarely seen are mentioned as included in this group. In a 

' Note v. Atti, p. 268. 

« Comptes Rendus, Uxiii. pp. 826-836, October 2, 187 1. 

S S 2 




The first su^a 
a td<^rani from 3 
Simla.' 



PROMIXENCES, SPOTS, AXD FACULjE. 629 



relation entre les protuberances ct les taches. De mon cot^, j'^tais note m. 

d<5jk arrive k la meme conclusion. Les 4, 5, et 6 Janvier, j'avais 

remarqud que, pres des taches, la raic noire C disparaissait, ce qui 
prouve que la lumi^re de ITiydrog^ne etait assez forte pour compenser 
Tabsorption du reste de Tatmosph^re solaire. Dans Tint^rieur des 
taches, on ne voit pas de raie brillante. C'est surtout dans la region 
des faculcs environnant la tache que la raic C s affaiblit, ou disparait 
compl6tement. 11 parait qu'on ne pourra jamais voir directement les 
raies brillantes sur le soleil lui-meme ; mais la disparition de la raie 
C suffit pour constater la presence d'une protuberance." » 

I could not accept this conclusion (see p. 493) in all its 
generality, and was driven to associate the prominences 
more with faculae than with spots, the prominences seen 
near spots being due to the facuLne with which the spots 
are always accompanied (sec p. 52i^\ the more as the pro- 
minences near spots are composed of jets at some period 
of their life. 

Young is also of this opinion : — ^ 

" From the observations above detailed, it is evident that the spots 
and prominences obey nearly the same law in respect to their distribu- 
tion on the solar surface ; but the prominences, which are far more ' 
numerous than the spots, approach nearer to the poles and are more 
frequently found on the equator. 

" I have never yet been able to watch a spot in its passage around 
the limb so that 1 could obser\c its ctTects on the chromosphere, but 
my present impression is that certain depressions from time to time 
obser\ed in the chromosphere (see Fig. 19', are due to spots directly 
under them. In only one case (No. 5. Nov. 4th) have I found a 
prominence very near a spot, and then only a small one. The spot 
referred to in connection with No. 9, Nov. 4th, was about 25" from the 
limb. Neither did spots make their apjjearance at or near the base of 
the great protuberances observed Oct. 7th. If they had they must 
have been seen on Oct. nth. Whether the prominences are connected 
with the facula: is a different question, and more likely, 1 think, to receive 
an affirmative answer.'* 

g; Still, though it was not true that all prominences were 

^ associated with spots, I pointed out, in April 1870, that 

the prominences which indicated the spectrum of the 
.^ lower vapours were generally associated with spots, in- 
^' eluding of course the facula platform (p. 518), while the 

cloud-like prominences ^^//rr/?//^ did not. My statement 

t' * Comptes Rcfidits, vol. Ixviii. p. 237, February 1869. 

y ' Journal 0/ the Franklin Institute^ No. 528, p. 423. 



i5 




total absence of pro! 
small jets. r 

" From many obi 
locality of the spots,] 

" I, Red stratum i 

" 2. On the nucli 
totally absent. 

"3. On the nuclei 
thev are confined to 
4. On the parts ; 
remarkable violence I 

"5. The jets neart 
but of other substanc 
bright lines. 

'■ 6. Among these b 
base, or in the lower 1 
and iron are often to 
red, one between B an 
extended by notable s 
shown very distinctly 1 

" 7. From lime to 

probably produced thi 
placements which are 

"8. In the locality I 
wards on the solar dis 
at times united in groi 

"9. The large jets 
developed and disapi 
quickly return to the s 

It will be percer 



PROMINENCES, SPOTS, AND FACUL^. 631 



surrounding a spot, and not with the nuclaei, is most prob- note m. 
ably correct. I have myself observed this, and can con- 
firm his observations.^ The statement, however, was 
violently attacked, which brought out the following reply 
from Respighi : — 

"In bringing forward these characteristics of the chromosphere and 
of the solar eruptions in the locality of the spots as results of a period 
of observation, it was certainly not my intention to establish unvarying 
laws and data, since I admitted that there are periodical variations in 
the solar eruptions, which would render it possible that these condi- 
tions also might be subject to great modifications and exceptions. 
Although I admitted this, I have been unjustly taxed with inaccuracy 
in my deductions from observations. If during other periods of 
observation some of these results failed, this ought to have been 
attributed, not to inaccuracy in my observations, but to the difference 
in the conditions prevailing in the two periods, differences in the sur- 
face and in the solar body. But this is not the spirit in which some 
astronomers work, some who, offering the greater part of these results 
as novelties originating from their own observations, have not scrupled 
to denounce as inaccurate those few results which did not appear to 
agree with these observations of theirs, made during a totally different 
period, and during a state of strong perturbation in the solar activity. 

" Thus they declared to be inaccurate the result of the depression and 
regularity of the red stratum in the locality of the spots, and of the 
temporary conditions of calm in the regions themselves, because during 
those observations the chromosphere appeared irregular, being covered 
with small jets and abounding with frequent eruptions. But this fact, 
instead of contradicting the results of my previous observations, merely 
shows that during the period of great .perturbation (in the locality of 
the spots, as well as throughout the whole of the solar surface) 
the solar activity is more energetic, and thence arise the irregularities 
of the chromosphere and the greater frequency of eruptions. This 
fact also gives evidence of the great influence which the eruptions 
have on the production, transformation, and displacement of the 
spots ; since during this period the appearance and disappearance 
of the spots were very frequent and the transformations continuous. 

" But in spite of the extraordinary activity in this period, not a few 
also cases occur of spots being on the limb or near to it, without being 
accompanied by notable eruptions, and only with the chromosphere 
bright, low, and with some small jets. Among the results of my early 
observations, that of the great depression of the red stratum on the 
nucleus of the spots has been pronounced inaccurate : from which 
depression I was led to suspect that perhaps on the nucleus the 
hydrogen does not exist in the luminous condition. 

" The argument brought against this conclusion of mine is founded 
on the fact that the red stratum in or over the nucleus of the spot is 
often sensibly elevated. 

* See also Young, in yountal of Franklin Institute^ No. 524, p. 142, 
Aug. 1S60, and quotation on page 629. 




It IS precisely i 

'Sroioobscneth, 

occasionally also b 

of the nucleus ; a 

great frequency o( 

stratum, 1 was abli 

visible atthflimh 

interval of quiet be 

'• Observing ihe s 

slit of the spectrosc 

projection on ihe Iji 

ine real inii-rniptior 

the limiis of rhc pi 

inio ibe sill Ihe sun- 

lomied by their spec 

dark, coinciding n-ii 

ascertained that, at 

the locality of the s 

of the chromosphere 

require exceedingly 1 

out observing the lare 

ance or disapfiearanc 

The fact, howe 
by Father Secchi, 
Academy this yea 

Je dirais qu'aprfc 
renconirSlachromospl 
une fois oil ceite env 
degrfe h^iioceniriques 
suts trfs, dispose i cnj 
corps inconnu, entre ni 



PROMINENCES, SPOTS, AND FACULjE, 633 



III. Prominences — Facula, 

From my observations, communicated to the Royal note m. 
Society in 1870, it will be seen that I am inclined to asso- 
ciate the prominences with the brighter parts of the faculae ; 
I do not hold to an absolute connection ; but I do hold 
to the lozenge as being the birth-place of prominences, 
although in the fainter prominences the pressure at work 
is too small, doubtless, to give rise to this appearance. 

Professor Respighi has stated his opinion on these points 
as follows. It will be seen that on one point we are not 
in agreement : — 

" Generally in the locality of the faculae the protuberances or the 
eruptions are more frequent and more developed, and it seems possible 
to conclude that the facula: are never unaccompanied by protuberances, 
while, however, there may be protuberances without faculse. 

" Although near the faculae are commonly found large protuberances, 
yet their positions are not proved to be so coincident as to allow us to 
consider them inseparable. The protuberances and the jets lie near 
the faculae, but constitute a phenomenon totally distinct from them. 

" On the other hand, this does not disprove that between these two 
phenomena the closest relations may exist ; nor does this exclude the 
idea that one of them may be necessarily dependent upon the other. 
1 have duly considered the concomitance of the faculx with the 
protuberances and with the eruptions, and it seems reasonable to 
suppose that either from the faculae the protuberance is produced, 
or vice versa. 

" Taking into consideration, however, that large protuberances are 
also seen near to the poles, in which regions there are no faculae, it 
seems to me more reasonable to suppose that the facula! may be a 
consequence of the protuberances, or rather of the solar eruptions ; 
from which might be produced in the photosphe